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/alt-svc.md

@@ -0,0 +1,28 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: alt-svc
+Arg: <file name>
+Protocols: HTTPS
+Help: Enable alt-svc with this cache file
+Added: 7.64.1
+Category: http
+Multi: append
+See-also:
+  - resolve
+  - connect-to
+Example:
+  - --alt-svc svc.txt $URL
+---
+
+# `--alt-svc`
+
+This option enables the alt-svc parser in curl. If the file name points to an
+existing alt-svc cache file, that gets used. After a completed transfer, the
+cache is saved to the file name again if it has been modified.
+
+Specify a "" file name (zero length) to avoid loading/saving and make curl
+just handle the cache in memory.
+
+If this option is used several times, curl loads contents from all the
+files but the last one is used for saving.