To render in Git webviews as-is, to make it easier to edit, verify,
and to sync up with curl.
- add options to not build the `.3` man pages:
- autotools: `--disable-docs`
- cmake: `LIBSSH2_BUILD_DOCS=OFF`
- building `.3` man pages requires Perl after this patch.
- drop `mansyntax` and the shell / `grep` / GNU `man` tool requirements with it.
- scripts and most logic were copied from curl.
- add `cd2nroff` from curl, with edits to relax curl-specific checks.
- used `nroff2cd` (from curl) to convert from `.3` to `.md`. Then
manually fixed copyrights, inline function references and a couple
of other things.
Credits-to: Daniel Stenberg
Ref: eefcc1bda4
Ref: https://github.com/curl/curl/pull/12730
Ref: https://github.com/libssh2/www/issues/25#issuecomment-3289431671Closes#1660
