docs: correct outdated Nightly contributor guide content

[killme2008]

What changed

Correct and complete the Nightly contributor guide while keeping it focused on component contracts, main execution paths, and contributor decisions rather than source navigation.

• Replace the early Metasrv design draft and speculative proto examples with current metadata, procedure, leader-election, heartbeat, failover, control-path, and table-route concepts.
• Explain both Flownode execution paths: batching is the primary path for aggregation and TQL workloads; streaming is retained for compatibility and deprecated for new workloads.
• Clarify Frontend request paths, Datanode Region dispatch, Mito WAL recovery boundaries, query planning and execution, partition routing and conservative pruning, and the conditional creation of Puffin index files.
• Turn the testing pages into a test-selection guide covering package tests, Sqlness, integration tests, compatibility tests, and result review.
• Document the current gRPC ingester contract: generated stubs, request representations, success and error metadata, bounded retries, and ambiguous delivery for unary and streaming writes.
• Add the missing Metasrv node-lease, recovery, and table-ID sequence endpoints, including their operational constraints.
• Use the official Apache Parquet file-layout diagram and identify its source.
• Keep the default selector recommendation, intentional overview links, and DeepWiki as a labeled secondary reference.
• Remove the obsolete Python Scripts pages after the implementation was removed.
• Tighten a small number of inflated or awkward sentences without expanding their technical scope.

The architecture and testing boundary follows established contributor documentation practice: explain abstractions, flows, invariants, and how to validate a change; leave implementation-level exploration to the source. No source directory or file-path navigation is added to the architecture pages.

Scope

• Documentation versions: Nightly, v1.2, v1.1, and v1.0. Each released version was gated on what that
  GreptimeDB release actually ships:
  • v1.2 (v1.2.0-beta.2) takes the change unchanged.
  • v1.1 (v1.1.4) drops the compatibility-test entry (tests/compatibility and cargo sqlness compat
    landed after that release) and x-greptime-err-retry-hint (not sent by v1.1).
  • v1.0 (v1.0.2) additionally omits the new Memtable page, because MemtableOptions there still has a
    PartitionTree variant, so the page's selection rules do not hold; links to it degrade to plain text.
    It also drops the pending-batching-flow sentence (defer_on_missing_source does not exist yet).
  • reference/sql/create.md and the Mito memtable configuration section change only in v1.1 and v1.2.
    v1.0.2 still has MitoConfig::memtable and accepts memtable.type=partition_tree, so the original
    text is correct there.
• Languages: English, Chinese

Verification

• DOC_LANG=en pnpm check:links
• DOC_LANG=zh pnpm check:links
• typos on all modified Markdown files
• git diff --check
• Verified version-sensitive claims against GreptimeDB main at 0cc83c4570, the current greptime-proto
  definitions, and tags v1.0.2, v1.1.4, and v1.2.0-beta.2 for the backported versions.
• Reviewed the complete diff relative to main; no released/0.x docs, generated outputs, or lockfiles changed. Two user-guide edits are intentional and in scope: user-guide/deployments-administration/configuration.md drops the removed [region_engine.mito.memtable] section, and reference/sql/create.md corrects the memtable.type values.

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• I updated navigation when the document structure changed. sidebars.ts adds the new Memtable page. The removed Python Scripts pages needed no navigation change because they were already excluded from the build.

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[WenyXu]

These comments clarify two Metasrv contracts that contributors must preserve: cache-invalidation fan-out and the separation between lease renewal and Region failure detection.

[killme2008]

f677db3d also contains four self-review fixes outside the commented lines:

• contributor-guide/overview.md: the previous sentence sent readers to CONTRIBUTING.md for build instructions, but that file has no prerequisites, toolchain, or build commands — the build guide is this repo's own Getting Started page. It now links Getting Started for building and scopes CONTRIBUTING.md to submission requirements.
• getting-started.md: restored Git (optional) and the 16GB/mold note. The build uses the git2 crate and open_repository tolerates a missing repository, so the git CLI is only needed to clone; the memory/mold guidance is still valid and has no replacement elsewhere.
• datanode/metric-engine.md: restored the indirect-routing paragraph. LogicalTableRouteValue still stores only physical_table_id, so migrating a physical Region rewrites one physical route instead of every logical route. That is a current invariant, not design rationale.
• PR description: corrected two statements that did not match the diff — user-guide/.../configuration.md and reference/sql/create.md are changed, and sidebars.ts adds the new Memtable page.

DOC_LANG=en pnpm check:links, DOC_LANG=zh pnpm check:links, and git diff --check all pass.

One item is deliberately left out of this PR: versioned_docs/version-1.1 and version-1.2 (and their Chinese pages) still document [region_engine.mito.memtable] and memtable.type = partition_tree. PartitionTreeMemtable was removed in greptimedb 7840aa1bb4, which shipped in v1.1.0, and unknown config keys are silently dropped, so a user following the v1.2 configuration page gets no error and no effect. Both released versions already have sst_format, so the Nightly wording backports cleanly. I will run the backport once this PR is approved.