[Cache] Document cache invalidation

[ack-cf]

Summary

Adds a cache invalidation guide covering API and dashboard workflows, origin revalidation, stale-content behavior, Cache Reserve, environment scope, limits, and verification. Links the guide from the existing purge and revalidation pages, updates the Cache Reserve purge explanation, and includes a draft changelog.

The guide follows the documentation style guide and canonical cache/guides/ structure, with MDX frontmatter, root-relative links, and the Steps and DashButton components. Background: Purge cache and Revalidation.

Draft for review — hold from merging until launch readiness is confirmed. Before merging:

• Confirm production endpoint acceptance, public API/SDK publication, and dashboard availability.
• Confirm plan/lifecycle availability and the documented Cache Reserve purge transition.
• Set the changelog filename and date to the actual publication date. The current date is a drafting date, not a confirmed launch date.

API examples use explicit curl requests while the invalidation schema publication is pending. Reassess the APIRequest component once the public schema contains these endpoints.

Validation

• git diff --check: passed.
• pnpm run format: passed; no unrelated tracked files changed.
• Targeted Prettier checks for the new guide, navigation page, and changelog: passed.
• pnpm run check:worker: passed.
• pnpm run check: blocked by four TypeScript errors in generated skills/turnstile-spin/templates/worker/ examples (ExportedHandler and Fetcher are unresolved), outside this diff.
• pnpm run build: emitted the new guide HTML, but exceeded the ten-minute local command timeout before final completion. A full successful build is not yet confirmed.

Screenshots (optional)

Review the deploy preview at /cache/guides/invalidate-cache/ before marking this PR ready.

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.