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_trace_sethandler.3

@@ -1,32 +0,0 @@
-.\" Copyright (C) The libssh2 project and its contributors.
-.\" SPDX-License-Identifier: BSD-3-Clause
-.TH libssh2_trace_sethandler 3 "15 Jan 2010" "libssh2" "libssh2"
-.SH NAME
-libssh2_trace_sethandler - set a trace output handler
-.SH SYNOPSIS
-.nf
-#include <libssh2.h>
-
-typedef void (*libssh2_trace_handler_func)(LIBSSH2_SESSION *session,
-                                           void *context,
-                                           const char *data,
-                                           size_t length);
-
-int
-libssh2_trace_sethandler(LIBSSH2_SESSION *session,
-                         void *context,
-                         libssh2_trace_handler_func callback);
-.fi
-.SH DESCRIPTION
-libssh2_trace_sethandler installs a trace output handler for your application.
-By default, when tracing has been switched on via a call to libssh2_trace(),
-all output is written to stderr. By calling this method and passing a
-function pointer that matches the libssh2_trace_handler_func prototype,
-libssh2 will call back as it generates trace output. This can be used to
-capture the trace output and put it into a log file or diagnostic window.
-This function has no effect unless libssh2 was built to support this option,
-and a typical "release build" might not.
-
-\fBcontext\fP can be used to pass arbitrary user defined data back into the callback when invoked.
-.SH AVAILABILITY
-Added in libssh2 version 1.2.3