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_scp_recv2.md

@@ -0,0 +1,49 @@
+---
+c: Copyright (C) The libssh2 project and its contributors.
+SPDX-License-Identifier: BSD-3-Clause
+Title: libssh2_scp_recv2
+Section: 3
+Source: libssh2
+See-also:
+  - libssh2_channel_open_ex(3)
+  - libssh2_session_init_ex(3)
+---
+
+# NAME
+
+libssh2_scp_recv2 - request a remote file via SCP
+
+# SYNOPSIS
+
+~~~c
+#include <libssh2.h>
+
+LIBSSH2_CHANNEL *
+libssh2_scp_recv2(LIBSSH2_SESSION *session, const char *path, struct_stat *sb);
+~~~
+
+# DESCRIPTION
+
+*session* - Session instance as returned by libssh2_session_init_ex(3)
+
+*path* - Full path and filename of file to transfer. That is the remote
+file name.
+
+*sb* - Populated with remote file's size, mode, mtime, and atime
+
+Request a file from the remote host via SCP.
+
+# RETURN VALUE
+
+Pointer to a newly allocated LIBSSH2_CHANNEL instance, or NULL on errors.
+
+# ERRORS
+
+*LIBSSH2_ERROR_ALLOC* - An internal memory allocation call failed.
+
+*LIBSSH2_ERROR_INVAL* - Invalid argument used in function call.
+
+*LIBSSH2_ERROR_SCP_PROTOCOL* -
+
+*LIBSSH2_ERROR_EAGAIN* - Marked for non-blocking I/O but the call would
+block.