CI: Add a `configure --with-docs=...` variant to require man pages rendered as HTML (but no heavy HTML docs)

[jimklimov]

Some platforms, e.g. NUT for Windows (when end-user has no MSYS2 ecosystem around it), do not benefit from local *roff renditions of manual pages - but may be happy to have HTML renditions. In other cases it may be nice to serve both *roff and HTML variants in a package, without the bigger docs (trade-offs in size and time allotted on CI servers, especially for ChangeLog).

This issue is about extending the strings accepted by configure script to support building man pages in a specified format (man, html, both, none, auto) optionally-independently of the other documentation. By default it should work as it does now (auto-mode may change), so adding HTML renditions if either html-single or html-chunked is enabled and possible.