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-urlencode.md

@@ -0,0 +1,51 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: data-urlencode
+Arg: <data>
+Help: HTTP POST data URL encoded
+Protocols: HTTP
+Added: 7.18.0
+Category: http post upload
+Multi: append
+See-also:
+  - data
+  - data-raw
+Example:
+  - --data-urlencode name=val $URL
+  - --data-urlencode =encodethis $URL
+  - --data-urlencode name@file $URL
+  - --data-urlencode @fileonly $URL
+---
+
+# `--data-urlencode`
+
+This posts data, similar to the other --data options with the exception
+that this performs URL-encoding.
+
+To be CGI-compliant, the <data> part should begin with a *name* followed
+by a separator and a content specification. The <data> part can be passed to
+curl using one of the following syntaxes:
+
+## content
+This makes curl URL-encode the content and pass that on. Just be careful
+so that the content does not contain any = or @ symbols, as that makes
+the syntax match one of the other cases below!
+
+## =content
+This makes curl URL-encode the content and pass that on. The preceding =
+symbol is not included in the data.
+
+## name=content
+This makes curl URL-encode the content part and pass that on. Note that
+the name part is expected to be URL-encoded already.
+
+## @filename
+This makes curl load data from the given file (including any newlines),
+URL-encode that data and pass it on in the POST.
+
+## name@filename
+This makes curl load data from the given file (including any newlines),
+URL-encode that data and pass it on in the POST. The name part gets an equal
+sign appended, resulting in *name=urlencoded-file-content*. Note that the
+name is expected to be URL-encoded already.