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/proxy-http2.md

@@ -0,0 +1,24 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: proxy-http2
+Tags: Versions HTTP/2
+Protocols: HTTP
+Added: 8.1.0
+Mutexed:
+Requires: HTTP/2
+Help: Use HTTP/2 with HTTPS proxy
+Category: http proxy
+Multi: boolean
+See-also:
+  - proxy
+Example:
+  - --proxy-http2 -x proxy $URL
+---
+
+# `--proxy-http2`
+
+Tells curl to try negotiate HTTP version 2 with an HTTPS proxy. The proxy might
+still only offer HTTP/1 and then curl sticks to using that version.
+
+This has no effect for any other kinds of proxies.