docs: introduce "curldown" for libcurl man page format

curldown is this new file format for libcurl man pages. It is markdown
inspired with differences:

- Each file has a set of

docs: introduce "curldown" for libcurl man page format

curldown is this new file format for libcurl man pages. It is markdown
inspired with differences:

- Each file has a set of leading headers with meta-data
- Supports a small subset of markdown
- Uses .md file extensions for editors/IDE/GitHub to treat them nicely
- Generates man pages very similar to the previous ones
- Generates man pages that still convert nicely to HTML on the website
- Detects and highlights mentions of curl symbols automatically (when
their man page section is specified)

tools:

- cd2nroff: converts from curldown to nroff man page
- nroff2cd: convert an (old) nroff man page to curldown
- cdall: convert many nroff pages to curldown versions
- cd2cd: verifies and updates a curldown to latest curldown

This setup generates .3 versions of all the curldown versions at build time.

CI:

Since the documentation is now technically markdown in the eyes of many
things, the CI runs many more tests and checks on this documentation,
including proselint, link checkers and tests that make sure we capitalize the
first letter after a period...

Closes #12730

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verify-examples.pl: fail verification on unescaped backslash

- Check that all backslashes in EXAMPLE are properly escaped.

eg manpage must always use `\\n` never `\n`.

This

verify-examples.pl: fail verification on unescaped backslash

- Check that all backslashes in EXAMPLE are properly escaped.

eg manpage must always use `\\n` never `\n`.

This is because the manpage requires we always double blackslash to show
a single backslash. Prior to this change an erroneous single backslash
would pass through and compile even though it would not show correctly
in the manpage.

Co-authored-by: Daniel Stenberg

Ref: https://github.com/curl/curl/pull/12588

Closes https://github.com/curl/curl/pull/12589

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tidy-up: whitespace

Closes #12484

docs: make all examples in all libcurl man pages compile

Closes #12448

verify-examples.pl: verify that all man page examples compile clean