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_knownhost_init.md

@@ -0,0 +1,41 @@
+---
+c: Copyright (C) Daniel Stenberg
+SPDX-License-Identifier: BSD-3-Clause
+Title: libssh2_knownhost_init
+Section: 3
+Source: libssh2
+See-also:
+  - libssh2_knownhost_add(3)
+  - libssh2_knownhost_check(3)
+  - libssh2_knownhost_free(3)
+---
+
+# NAME
+
+libssh2_knownhost_init - init a collection of known hosts
+
+# SYNOPSIS
+
+~~~c
+#include <libssh2.h>
+
+LIBSSH2_KNOWNHOSTS *
+libssh2_knownhost_init(LIBSSH2_SESSION *session);
+~~~
+
+# DESCRIPTION
+
+Init a collection of known hosts for this session. Returns the handle to an
+internal representation of a known host collection.
+
+Call **libssh2_knownhost_free(3)** to free the collection again after you are
+doing using it.
+
+# RETURN VALUE
+
+Returns a handle pointer or NULL if something went wrong. The returned handle
+is used as input to all other known host related functions libssh2 provides.
+
+# AVAILABILITY
+
+Added in libssh2 1.2