Documentation: Refactor Branding Your Installation > Custom Header

[j-n-c]

As discussed in issue #8591, Branding Your Installation > Custom Header section in the documentation should be refactored to further explain what changes can be made and what to expect after customizing the page header.

[j-n-c]

@pdurbin could You please assgn this issue to me?

[pdurbin]

@j-n-c done! Thanks for working on this!