docs: add meaningful implementation checklist

[silentgeckoaudit3801]

Closes #126.

Summary

• Add a meaningful implementation checklist for AnchorKit contributors and reviewers.
• Cover full issue scope, cross-package impact, tests/docs, acceptance criteria, and small incomplete examples.
• Link the checklist from README and reference it from the PR template summary guidance.

Verification

• Docs/template-only change; no runtime tooling required.
• Compared against upstream main: 3 files changed, 0 commits behind.

[silentgeckoaudit3801]

Thanks for merging this. Is there an official GrantFox reward claim or payment step I should follow for the merged Maybe Rewarded issue #126?