Doxygen: simplify logic, remove vendored header/footer/css files

[ManifoldFR]

Following the discussion on my issue #737, here is my proposed change.

I will propose an update to the vendored MathJax in a subsequent PR.

Resolves #737

cc @nim65s @jmirabel

[jmirabel]

Is it supported to use doxygen without providing a header, footer and css ?
In the past, it used not to be the case.

[ManifoldFR]

Is it supported to use doxygen without providing a header, footer and css ? In the past, it used not to be the case.

Yes! From what I'm seeing in the docs:

The HTML_HEADER tag can be used to specify a user-defined HTML header file for each generated HTML page. If the tag is left blank Doxygen will generate a standard header.

I'm not aware that it was ever not the case however. I started looking around in the changelog and at some resolved issues on doxygen's repo from like over a decade ago (like doxygen/doxygen#4079), and I'm not finding anything.

[ManifoldFR]

There is a mention of a default CSS stylesheet in the changelog dating back to 1.5.7 (in 2008)