feat(generate): add generate llms command for per-project llms.txt

[dacharyc]

Summary

Adds a new generate llms command that produces an llms.txt file for each documentation project, supporting a progressive-disclosure setup: a master llms.txt acts as a sitemap linking to each project's own llms.txt.

For each project's current version (and non-versioned projects), the command:

• enumerates pages and extracts each page's title (H1) and meta :description:
• resolves the production URL via the existing resolver, appending .md
• writes <output-dir>/<project>/llms.txt in the format:
  - [Page Title](https://www.mongodb.com/docs/manual/core/document.md): Description.
• prints a per-project character-count summary (with and without descriptions), flagging files over the 50,000-character llms.txt guideline

Behavior details

• Root landing pages use the <root>/index.md form (the <root>.md form 404s in production; <root>/index.md is the real markdown).
• Substitutions: snooty {+name+} references in titles and descriptions are resolved from the project's snooty.toml [constants].
• Missing descriptions omit the trailing : description.
• Excluded: includes/ and code-examples/ directories; deprecated app-services and realm projects; non-project dirs (404, docs-platform, meta, table-of-contents).
• --no-descriptions flag omits descriptions from written files (useful for oversized projects / iterating on docs).

Flags

--output-dir, --for-project, --no-descriptions, --base-url

Findings from a full run (46 projects)

6 projects exceed 50k with descriptions (atlas, atlas-cli, cloud-manager, manual, mongocli, ops-manager); none of them drop under 50k by removing descriptions alone, so they'll need per-section splitting. The other 40 fit comfortably with descriptions.

Changes

• New: commands/generate/{generate.go,llms/*}, internal/rst/{meta_parser.go,page_title.go}
• Modified: main.go (register command), internal/snooty/snooty.go (constants + ResolveSubstitutions), README.md
• Tests for meta/title parsers, substitution resolution, URL/render helpers

Test plan

• go build ./... ✅
• go test ./... ✅ (all packages green)
• Verified root-index .md URLs return 200 live; confirmed 0 remaining {+...+} refs across all generated files

[cbullinger]

✅