Replace the README example and drop stale timing caveats from the docs

[janicduplessis]

Description

Documentation cleanup after 0.1.0:

• README usage example. The example called console.log inside a map that returned nulls, so it rendered nothing. It now splits content into two panes around a vertical division and falls back to one pane when there is none (typechecked against src/index.tsx).
• Version notes. Removed "available starting with 0.1.0-alpha.3" from the README, api.md and performance.md, since 0.1.0 is the stable release.
• Reanimated/Worklets section. The "Readiness-gated animated content" section (Worklets __requestMapperRunFinalizer, example-only FIFO patch) is gone from the README, usage.md and the performance.md link that pointed at its anchor. performance.md's "Synchronous delivery and animation" heading is now "Synchronous delivery".
• iOS timing caveat. This is now documented once, under "Measurement timing" in platforms.md, with a one-line pointer in the README. iOS measures in layoutSubviews, and React Native processes a synchronous event requested there one frame late unless it includes react/react-native#58530. The docs called that PR "unmerged". It is now merged upstream (b439ef7) but not yet in a React Native release, and platforms.md and example.md now say so.

Left as is: historical release notes in docs/releases/, which still mention the Worklets workaround, and the "unmerged" wording in docs/workflow.md and the example Podfile comment.

Test plan

• pnpm run docs:build passes with the #readiness-gated-animated-content anchor and its only link removed.
• git grep -i "alpha.3\|worklets\|reanimated\|readiness-gated" -- README.md website/docs returns nothing.