docs/cmdline: change to .md for cmdline docs

- switch all invidual files documenting command line options into .md,
   as the documentation is now markdown-looking.

 - made the parser treat 4-space indents as quotes

 - switch to building the curl.1 manpage using the "mainpage.idx" file,
   which lists the files to include to generate it, instead of using the
   previous page-footer/headers. Also, those files are now also .md
   ones, using the same format. I gave them underscore prefixes to make
   them sort separately:
   _NAME.md, _SYNOPSIS.md, _DESCRIPTION.md, _URL.md, _GLOBBING.md,
   _VARIABLES.md, _OUTPUT.md, _PROTOCOLS.md, _PROGRESS.md, _VERSION.md,
   _OPTIONS.md, _FILES.md, _ENVIRONMENT.md, _PROXYPREFIX.md,
   _EXITCODES.md, _BUGS.md, _AUTHORS.md, _WWW.md, _SEEALSO.md

 - updated test cases accordingly

Closes #12751

docs/cmdline-opts/data-binary.md

@@ -0,0 +1,31 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: data-binary
+Arg: <data>
+Help: HTTP POST binary data
+Protocols: HTTP
+Category: http post upload
+Added: 7.2
+Multi: append
+See-also:
+  - data-ascii
+Example:
+  - --data-binary @filename $URL
+---
+
+# `--data-binary`
+
+This posts data exactly as specified with no extra processing whatsoever.
+
+If you start the data with the letter @, the rest should be a filename. Data
+is posted in a similar manner as --data does, except that newlines and
+carriage returns are preserved and conversions are never done.
+
+Like --data the default content-type sent to the server is
+application/x-www-form-urlencoded. If you want the data to be treated as
+arbitrary binary data by the server then set the content-type to octet-stream:
+-H "Content-Type: application/octet-stream".
+
+If this option is used several times, the ones following the first append
+data as described in --data.