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.md

@@ -0,0 +1,49 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: data
+Short: d
+Arg: <data>
+Help: HTTP POST data
+Protocols: HTTP MQTT
+Mutexed: form head upload-file
+Category: important http post upload
+Added: 4.0
+Multi: append
+See-also:
+  - data-binary
+  - data-urlencode
+  - data-raw
+Example:
+  - -d "name=curl" $URL
+  - -d "name=curl" -d "tool=cmdline" $URL
+  - -d @filename $URL
+---
+
+# `--data`
+
+Sends the specified data in a POST request to the HTTP server, in the same way
+that a browser does when a user has filled in an HTML form and presses the
+submit button. This makes curl pass the data to the server using the
+content-type application/x-www-form-urlencoded. Compare to --form.
+
+--data-raw is almost the same but does not have a special interpretation of
+the @ character. To post data purely binary, you should instead use the
+--data-binary option. To URL-encode the value of a form field you may use
+--data-urlencode.
+
+If any of these options is used more than once on the same command line, the
+data pieces specified are merged with a separating &-symbol. Thus, using
+'-d name=daniel -d skill=lousy' would generate a post chunk that looks like
+'name=daniel&skill=lousy'.
+
+If you start the data with the letter @, the rest should be a file name to
+read the data from, or - if you want curl to read the data from stdin. Posting
+data from a file named 'foobar' would thus be done with --data @foobar. When
+--data is told to read from a file like that, carriage returns and newlines
+are stripped out. If you do not want the @ character to have a special
+interpretation use --data-raw instead.
+
+The data for this option is passed on to the server exactly as provided on the
+command line. curl does not convert, change or improve it. It is up to the
+user to provide the data in the correct form.