chore(docs): add extensions link to nav links

[UziTech]

Marked version: docs

Description

Add extensions link to navigation links

/using_pro#extensions

Contributor

• Test(s) exist to ensure functionality and minimize regression (if no tests added, list tests covering this PR); or,
• no tests required for this PR.
• If submitting new feature, it has been documented in the appropriate places.

Committer

In most cases, this should be a different person than the contributor.

• CI is green (no forced merge required).
• Squash and Merge PR following conventional commit guidelines.

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[github-actions]

🎉 This PR is included in version 2.1.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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