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_callback_set.3

@@ -1,31 +0,0 @@
-.\" Copyright (C) The libssh2 project and its contributors.
-.\" SPDX-License-Identifier: BSD-3-Clause
-.TH libssh2_session_callback_set 3 "1 Jun 2007" "libssh2 0.15" "libssh2"
-.SH NAME
-libssh2_session_callback_set - set a callback function
-.SH SYNOPSIS
-.nf
-#include <libssh2.h>
-
-void *
-libssh2_session_callback_set(LIBSSH2_SESSION *session,
-                             int cbtype, void *callback);
-.fi
-.SH DESCRIPTION
-This function is \fBDEPRECATED\fP in 1.11.1. Use the
-\fIlibssh2_session_callback_set2(3)\fP function instead!
-
-This implementation is expecting and returning a data pointer for callback
-functions.
-
-For the details about the replacement function, see
-.BR libssh2_session_callback_set2(3)
-which is expecting and returning a function pointer.
-
-.SH RETURN VALUE
-Pointer to previous callback handler. Returns NULL if no prior callback
-handler was set or the callback type was unknown.
-.SH SEE ALSO
-.BR libssh2_session_callback_set2(3)
-.BR libssh2_session_init_ex(3)
-.BR libssh2_agent_sign(3)