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/location.md

@@ -0,0 +1,40 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: location
+Short: L
+Help: Follow redirects
+Protocols: HTTP
+Category: http
+Added: 4.9
+Multi: boolean
+See-also:
+  - resolve
+  - alt-svc
+Example:
+  - -L $URL
+---
+
+# `--location`
+
+If the server reports that the requested page has moved to a different
+location (indicated with a Location: header and a 3XX response code), this
+option makes curl redo the request on the new place. If used together with
+--include or --head, headers from all requested pages are shown.
+
+When authentication is used, curl only sends its credentials to the initial
+host. If a redirect takes curl to a different host, it does not get the
+user+password pass on. See also --location-trusted on how to change this.
+
+Limit the amount of redirects to follow by using the --max-redirs option.
+
+When curl follows a redirect and if the request is a POST, it sends the
+following request with a GET if the HTTP response was 301, 302, or 303. If the
+response code was any other 3xx code, curl resends the following request using
+the same unmodified method.
+
+You can tell curl to not change POST requests to GET after a 30x response by
+using the dedicated options for that: --post301, --post302 and --post303.
+
+The method set with --request overrides the method curl would otherwise select
+to use.