docs: convert man page sources to "curldown" format

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-3289431671

Closes #1660

docs/libssh2_session_init.3

@@ -1,21 +0,0 @@
-.\" Copyright (C) The libssh2 project and its contributors.
-.\" SPDX-License-Identifier: BSD-3-Clause
-.TH libssh2_session_init 3 "20 Feb 2010" "libssh2 1.2.4" "libssh2"
-.SH NAME
-libssh2_session_init - convenience macro for \fIlibssh2_session_init_ex(3)\fP calls
-.SH SYNOPSIS
-.nf
-#include <libssh2.h>
-
-LIBSSH2_SESSION *
-libssh2_session_init(void);
-.fi
-.SH DESCRIPTION
-This is a macro defined in a public libssh2 header file that is using the
-underlying function \fIlibssh2_session_init_ex(3)\fP.
-.SH RETURN VALUE
-See \fIlibssh2_session_init_ex(3)\fP
-.SH ERRORS
-See \fIlibssh2_session_init_ex(3)\fP
-.SH SEE ALSO
-.BR libssh2_session_init_ex(3)