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

.github/workflows/linkcheck.yml

@@ -31,6 +31,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        name: checkout
+
+      - name: trim the cmdline docs markdown files
+        run: find docs/cmdline-opts -name "*.md" ! -name "_*" ! -name MANPAGE.md | xargs -n1 ./.github/scripts/cleancmd.pl
+
       - uses: gaurav-nelson/github-action-markdown-link-check@v1
         with:
           use-quiet-mode: 'yes'

.github/workflows/spellcheck.yml

@@ -57,6 +57,12 @@ jobs:
           perl -pi -e 's/\-\-[\a-z0-9-]*//ig' docs/curl.md
           perl -pi -e 's!https://[a-z0-9%/.-]*!!ig' docs/curl.md
 
+      - name: trim the cmdline docs markdown files
+        run: find docs/cmdline-opts -name "*.md" ! -name "_*" ! -name MANPAGE.md | xargs -n1 ./.github/scripts/cleancmd.pl
+
+      - name: trim the cmdline docs markdown _*.md files
+        run: find docs/cmdline-opts -name "_*.md" | xargs -n1 ./.github/scripts/cleancmd.pl --no-header
+
       - name: setup the custom wordlist
         run: grep -v '^#' .github/scripts/spellcheck.words >  wordlist.txt