Updated the URL for Get involved link in the navbar

[MukulKolpe]

Fixes

Fixes #1988 by @obulat

Description

Changed the URL for Get involved link in the new header navbar from the link to the GitHub repository to Openverse Make Handbook.

Testing Instructions

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[dhruvkb]

This is good! The Make site is a considerably better place to start for new contributors as compared to the WordPress/openverse repository.

[obulat]

Awesome, @MukulKolpe ! Thank you for your contribution.