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_agent_get_identity.md

@@ -0,0 +1,50 @@
+---
+c: Copyright (C) Daiki Ueno
+SPDX-License-Identifier: BSD-3-Clause
+Title: libssh2_agent_get_identity
+Section: 3
+Source: libssh2
+See-also:
+  - libssh2_agent_list_identities(3)
+  - libssh2_agent_userauth(3)
+---
+
+# NAME
+
+libssh2_agent_get_identity - get a public key off the collection of public keys managed by ssh-agent
+
+# SYNOPSIS
+
+~~~c
+#include <libssh2.h>
+
+int
+libssh2_agent_get_identity(LIBSSH2_AGENT *agent,
+                           struct libssh2_agent_publickey **store,
+                           struct libssh2_agent_publickey *prev);
+~~~
+
+# DESCRIPTION
+
+*libssh2_agent_get_identity(3)* allows an application to iterate
+over all public keys in the collection managed by ssh-agent.
+
+*store* should point to a pointer that gets filled in to point to the
+public key data.
+
+*prev* is a pointer to a previous 'struct libssh2_agent_publickey'
+as returned by a previous invoke of this function, or NULL to get the
+first entry in the internal collection.
+
+# RETURN VALUE
+
+Returns 0 if everything is fine and information about a host was stored in
+the *store* struct.
+
+Returns 1 if it reached the end of public keys.
+
+Returns negative values for error
+
+# AVAILABILITY
+
+Added in libssh2 1.2