Update about page to include external sources information

[anton202]

Fixes

Fixes #1989 by @obulat

Description

This PR adds the external sources page text to about page.
the extrenal sources text in the about page is displayed only if the new header is available.

Testing Instructions

1. go to the /about page. make sure there is no exteranl source text displayed.
2. go to /preferences. enable the new header. got to /about page. make sure that the external sources text is displayed

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.

Developer Certificate of Origin

Developer Certificate of Origin

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[obulat]

These changes are great!
We'll need to update the snapshots for Virtual regression tests. Please let me know if you need any help, or if you want me to update the snapshots, I'd be happy to help!

[anton202]

thx. i`d like to try it myself. i`ll let you know if i have any questions :)

[obulat]

The snapshots are awesome, thank you, @anton202 !

[sarayourfriend]

LGTM! I noticed one discrepancy in the text but we could create an issue to fix that instead of doing it here. I'll leave that up to Olga.