New keyword for man page/documentation installation

[newtonne]

In a similar vein to #56963, I think Portfiles could be cleaned up if there were keywords that could install man pages and documentation, rather than repeating the same patterns.

For example, something like:

documentation {*}[glob ${workpath}/${distname}/docs/_build/singlehtml/*]

Instead of:

set docdir ${destroot}${prefix}/share/doc/${name}
xinstall -d ${docdir}
file copy {*}[glob ${workpath}/${distname}/docs/_build/singlehtml/*] ${docdir}

A sensible default would need to be chosen for the directory to install the files to - probably ${prefix}/share/doc/${name} for documentation.

As for man pages, there could be different keywords for different man page sections. For example:

man1 ${worksrcpath}/docs/_build/man/borg.1

would install the man page to ${prefix}/share/man/man1/.