Replace usage of `\htmlonly` with `<div>`

[albert-github]

The HTML <div> construct with attributes is supported by doxygen (in HTML, on other places it is ignored) and can theerfore be removed.

• the advantage is that a mismatch of <div> / </div> is notice (e.g. when they are in different ALIASES and one of the 2 has been forgotten, see also #8560)
• the ALIASES: cgalModifBegin and cgalModifEnd are not used (anymore) and are therefore removed

[afabri]

What do you mean with that cgalModifBegin are not used. They do not appear in the master branch but they are useful when proposing changes in the manual. So they are just needed for "development".

[albert-github]

Reinstated the"modif" ALIASES.

• what to do with the \latexonly part ?
• interesting question is are these 2 ALIASES filtered out when a release is made so they won't accidentally slip into a release?

[afabri]

I think we could give up \latexonly as we have no ambition to create something like pdf. @MaelRL any opinion?

[sloriot]

Successfully tested in CGAL-6.1-Ic-50