Correcting stylesheet name

[albert-github]

The name stylesheet.css is replaced by a better name cgal_stylesheet.css a while ago, but apparently these 2 files were not updated.

[lrineau]

I am surprised that https://doc.cgal.org/latest/Manual/how_to_cite_cgal.html does use cgal_stylesheet.css. Do you know why?

[albert-github]

As far as I can quickly see there is only one place where the stylsheet is used i.e. <h1>, but I could imagine that is would be used at other places as well (the <a>, the <pre>)

At the moment the how_to_cite.html (not the how_to_cite_cgal is the file we are talking about) is just a "stand alone" page that is called from how_to_cit_cgal.html through the bib links without the "CGAL header" and without the tree view. I think it would also be worthwhile to adjust this (but that would be something for a different enhancement issue / pull request).

At the moment though the usage of stylesheet.css is not correct and should either be removed or replaced as suggested in this proposed PR.

[sloriot]

Successfully tested in CGAL-6.0-Ic-86