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_init.md

@@ -0,0 +1,38 @@
+---
+c: Copyright (C) The libssh2 project and its contributors.
+SPDX-License-Identifier: BSD-3-Clause
+Title: libssh2_init
+Section: 3
+Source: libssh2
+See-also:
+  - libssh2_exit(3)
+---
+
+# NAME
+
+libssh2_init - global library initialization
+
+# SYNOPSIS
+
+~~~c
+#include <libssh2.h>
+
+#define LIBSSH2_INIT_NO_CRYPTO 0x0001
+
+int
+libssh2_init(int flags);
+~~~
+
+# DESCRIPTION
+
+Initialize the libssh2 functions. This typically initialize the
+crypto library. It uses a global state, and is not thread safe -- you
+must make sure this function is not called concurrently.
+
+# RETURN VALUE
+
+Returns 0 if succeeded, or a negative value for error.
+
+# AVAILABILITY
+
+Added in libssh2 1.2.5