docs: capitalize React Actions references

[lil-goat]

Summary

Fixes #6713 — Capitalize React concepts in docs
Source issue: #6713

This PR covers the remaining Actions concept scope for issue #6713 by
capitalizing React-specific references to Actions in docs prose and headings.

Scope

• Updated React Actions concept references in:
  • src/content/reference/react/hooks.md
  • src/content/reference/react/useTransition.md
  • src/content/reference/react/useActionState.md
  • src/content/reference/rsc/server-functions.md
  • src/content/learn/rsc-sandbox-test.md
• Intentionally did not rename code identifiers like action, formAction, or
  actions.js.
• Did not touch blog or community pages.

Verification

• Reviewed the diff manually to keep the changes limited to React-concept prose.
• Re-ran targeted searches to avoid changing code examples, prop names, or file names.

Blockers

• yarn install --frozen-lockfile did not complete within this session, so full
  automated validation is still pending.
• node scripts/headingIdLinter.js ... currently requires repo dependencies
  (for example github-slugger) that were not yet installed.

Notes

• Opening as a draft so automated lint / heading checks can be completed once
  dependency installation finishes locally.

[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

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This PR introduced no changes to the JavaScript bundle! 🙌