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_get_exit_signal.md

@@ -0,0 +1,52 @@
+---
+c: Copyright (C) The libssh2 project and its contributors.
+SPDX-License-Identifier: BSD-3-Clause
+Title: libssh2_channel_get_exit_signal
+Section: 3
+Source: libssh2
+See-also:
+---
+
+# NAME
+
+libssh2_channel_get_exit_signal - get the remote exit signal
+
+# SYNOPSIS
+
+~~~c
+#include <libssh2.h>
+
+int
+libssh2_channel_get_exit_signal(LIBSSH2_CHANNEL *channel,
+                                char **exitsignal, size_t *exitsignal_len,
+                                char **errmsg, size_t *errmsg_len,
+                                char **langtag, size_t *langtag_len);
+~~~
+
+# DESCRIPTION
+
+*channel* - Closed channel stream to retrieve exit signal from.
+
+*exitsignal* - If not NULL, is populated by reference with the exit signal
+(without leading "SIG"). Note that the string is stored in a newly allocated
+buffer. If the remote program exited cleanly, the referenced string pointer
+will be set to NULL.
+
+*exitsignal_len* - If not NULL, is populated by reference with the length
+of exitsignal.
+
+*errmsg* - If not NULL, is populated by reference with the error message
+(if provided by remote server, if not it will be set to NULL). Note that the
+string is stored in a newly allocated buffer.
+
+*errmsg_len* - If not NULL, is populated by reference with the length of errmsg.
+
+*langtag* - If not NULL, is populated by reference with the language tag
+(if provided by remote server, if not it will be set to NULL). Note that the
+string is stored in a newly allocated buffer.
+
+*langtag_len* - If not NULL, is populated by reference with the length of langtag.
+
+# RETURN VALUE
+
+Numeric error code corresponding to the the Error Code constants.