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_channel_direct_tcpip.md

@@ -0,0 +1,36 @@
+---
+c: Copyright (C) The libssh2 project and its contributors.
+SPDX-License-Identifier: BSD-3-Clause
+Title: libssh2_channel_direct_tcpip
+Section: 3
+Source: libssh2
+See-also:
+  - libssh2_channel_direct_tcpip_ex(3)
+---
+
+# NAME
+
+libssh2_channel_direct_tcpip - convenience macro for *libssh2_channel_direct_tcpip_ex(3)* calls
+
+# SYNOPSIS
+
+~~~c
+#include <libssh2.h>
+
+LIBSSH2_CHANNEL *
+libssh2_channel_direct_tcpip(LIBSSH2_SESSION *session,
+                             const char *host, int port);
+~~~
+
+# DESCRIPTION
+
+This is a macro defined in a public libssh2 header file that is using the
+underlying function *libssh2_channel_direct_tcpip_ex(3)*.
+
+# RETURN VALUE
+
+See *libssh2_channel_direct_tcpip_ex(3)*
+
+# ERRORS
+
+See *libssh2_channel_direct_tcpip_ex(3)*