docs/cmdline: refer to --show-headers instead of --include

As it is the new version of the option that is easier to understand what
it does by name.

Follow-up to 82c53f821fa3fd4

docs/cmdline: refer to --show-headers instead of --include

As it is the new version of the option that is easier to understand what
it does by name.

Follow-up to 82c53f821fa3fd40
Closes #14497

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docs/cmdline-opts: update see-also to use show-headers

Since --include is now deprecated since 82c53f821fa3fd4

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

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

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