[Docs] Fix: Two broken links to the packages reference API and to blocks docs

[jorgefilipecosta]

The links /docs/reference-guides/packages/ and /docs/reference-guides/packages/packages-blocks.md don't link to any valid destination. This PR fixes both links to point to the correct destinations.

Testing Instructions

Using GitHub UI I went to https://github.com/WordPress/gutenberg/blob/40e003d85a06b44037beecb8b5bd7aa253fe7e67/docs/getting-started/fundamentals/javascript-in-the-block-editor.md and verified both links https://github.com/WordPress/gutenberg/blob/40e003d85a06b44037beecb8b5bd7aa253fe7e67/docs/reference-guides/packages.md and https://github.com/WordPress/gutenberg/blob/40e003d85a06b44037beecb8b5bd7aa253fe7e67/packages/blocks/README.md work while on https://github.com/WordPress/gutenberg/blob/9437f7e73f9c6c9f831f2a26a555dfa8fddb3d61/docs/getting-started/fundamentals/javascript-in-the-block-editor.md (current trunk commit) https://github.com/WordPress/gutenberg/blob/9437f7e73f9c6c9f831f2a26a555dfa8fddb3d61/docs/reference-guides/packages and https://github.com/WordPress/gutenberg/blob/9437f7e73f9c6c9f831f2a26a555dfa8fddb3d61/docs/reference-guides/packages/packages-blocks.md don't work.

[github-actions]

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Co-authored-by: jorgefilipecosta <[email protected]>
Co-authored-by: shail-mehta <[email protected]>

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[github-actions]

Flaky tests detected in 40e003d.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/12302462831
📝 Reported issues:

• [Flaky Test] Navigates the items list via UP/DOWN arrow keys #64334 in /test/e2e/specs/site-editor/dataviews-list-layout-keyboard.spec.js
• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js

[shail-mehta]

Nice Catch!