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/variable.md

@@ -0,0 +1,58 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: variable
+Arg: <[%]name=text/@file>
+Help: Set variable
+Category: curl
+Added: 8.3.0
+Multi: append
+See-also:
+  - config
+Example:
+  - --variable name=smith $URL
+---
+
+# `--variable`
+
+Set a variable with "name=content" or "name@file" (where "file" can be stdin
+if set to a single dash (-)). The name is a case sensitive identifier that
+must consist of no other letters than a-z, A-Z, 0-9 or underscore. The
+specified content is then associated with this identifier.
+
+Setting the same variable name again overwrites the old contents with the new.
+
+The contents of a variable can be referenced in a later command line option
+when that option name is prefixed with "--expand-", and the name is used as
+"{{name}}" (without the quotes).
+
+--variable can import environment variables into the name space. Opt to either
+require the environment variable to be set or provide a default value for the
+variable in case it is not already set.
+
+--variable %name imports the variable called 'name' but exits with an error if
+that environment variable is not already set. To provide a default value if
+the environment variable is not set, use --variable %name=content or
+--variable %name@content. Note that on some systems - but not all -
+environment variables are case insensitive.
+
+When expanding variables, curl supports a set of functions that can make the
+variable contents more convenient to use. You apply a function to a variable
+expansion by adding a colon and then list the desired functions in a
+comma-separated list that is evaluated in a left-to-right order. Variable
+content holding null bytes that are not encoded when expanded, causes an
+error.
+
+Available functions:
+
+## trim
+removes all leading and trailing white space.
+
+## json
+outputs the content using JSON string quoting rules.
+
+## url
+shows the content URL (percent) encoded.
+
+## b64
+expands the variable base64 encoded