docs: use official cache actions

[Samielakkad]

Closes #827

Summary

• replace the third-party artifact download/upload flow with the official actions/cache/restore and actions/cache/save actions
• use per-run and per-attempt primary keys plus a stable restore prefix so the mutable Renovate cache is restored from the newest prior run and saved under a fresh immutable key
• retain the required UID 12021 ownership correction, gated on an actual exact or partial restore
• preserve the existing enabled, disabled, and reset workflow-dispatch behavior
• remove the manual tar archive and update the stale checkout action in the example

The unique key is necessary because GitHub cache entries are immutable. Including github.run_attempt also lets a rerun save an updated cache without colliding with the previous attempt.

Validation

• extracted cache workflow parses as YAML
• pnpm lint passes, including ESLint and repository-pinned Prettier
• verified the documented inputs and cache-hit / cache-matched-key outputs against actions/cache v6.1.0
• git diff --check passes