Config Projects Extra Fields: link to the doc (like User)

Summary:
Add a documentation link in a specific Config page, that is:

Config > Setting > projects.custom-field-definitions

Comparison example:

This change is a boring follow-up of this one:

https://we.phorge.it/D25507

Test Plan:
Visit the page and click on the new link:

• /config/edit/projects.custom-field-definitions/

Visit this page that was the inspirational page:

• /config/edit/user.custom-field-definitions/

Check that no nuclear implosion is raised.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25552