docs: mark testnet-02 as retired and document current networks

[subheeksh5599]

Overview

testnet-02 was retired in January 2026 and its endpoints no longer resolve (verified NXDOMAIN), but the README still told new users to set CFG_PRESET=testnet-02 as the example preset — sending every new integrator to a dead network first.

Changes:

• README: the CFG_PRESET example no longer points at the retired network; added a short Networks section documenting the current public networks (preview, preprod, mainnet), the docs page for endpoints/migration, and the status of the presets shipped in this repo.
• .envrc.testnet-02: marked as RETIRED (kept for reference; no behavior change).

No config or behavior changes — documentation only. The default CFG_PRESET in .envrc is intentionally left untouched: the only other in-repo preset (qanet) targets an internal network, and updated presets for the public networks need the newer chain-config schema and node image, which is a maintainer decision (tracked in midnightntwrk/midnight-docs#1172).

Submission Checklist

• Useful pull request description
• Tests are provided (if possible) — docs-only change, existing CI covers config validation
• Key commits have useful messages
• All check jobs of the CI have succeeded — awaiting CI run
• Self-reviewed the diff
• Reviewer requested
• Update README.md file (if relevant)
• Update documentation (if relevant)
• No new todos introduced

Links

• Contributes to ~80 stale testnet-02 references across org repos, six months after the Preview transition midnight-docs#1172 (stale testnet-02 references across org repos, names this repo's README and .envrc)
• References Midnight Node with image 0.18.0-rc.7 and the environment is testnet-02 can't running #76 (testnet-02 preset cannot run with newer node images)