GHA: silence proselint warnings and an error

Fix new issues found by `proselint`.

Also:
- silence this technical warning:
```
:0: DeprecationWarning: /home/runner/.p

GHA: silence proselint warnings and an error

Fix new issues found by `proselint`.

Also:
- silence this technical warning:
```
:0: DeprecationWarning: /home/runner/.proselintrc was found instead of a JSON file. Rename to /home/runner/.proselintrc.json.
```
- fix an input filename.
`proselints` fails now if an input file is missing.

Reported-by: Jay Satiro
Bug: https://github.com/curl/curl/pull/15291#issuecomment-2410505100
Closes #15293

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curldown: make 'added-in:' a mandatory header field

- generate AVAILABILITY manpage sections automatically - for consistent
wording

- allows us to double-check against other d

curldown: make 'added-in:' a mandatory header field

- generate AVAILABILITY manpage sections automatically - for consistent
wording

- allows us to double-check against other documumentation (symbols-in-versions
etc)

- enables proper automation/scripting based on this data

- lots of them were wrong or missing in the manpages

- several of them repeated (sometimes mismatching) backend support info

Add test 1488 to verify "added-in" version numbers against
symbols-in-versions.

Closes #14217

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docs/libcurl: generate PROTOCOLS from meta-data

Remove the PROTOCOLS section from the source files completely and
instead generate them based on the header data in the curldown files.

docs/libcurl: generate PROTOCOLS from meta-data

Remove the PROTOCOLS section from the source files completely and
instead generate them based on the header data in the curldown files.

It also generates TLS backend information for options marked for TLS as
protocol.

Closes #13175

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docs: make each libcurl man specify protocol(s)

The mandatory header now has a mandatory list of protocols for which the
manpage is relevant.

Most man pages already has a "PROTO

docs: make each libcurl man specify protocol(s)

The mandatory header now has a mandatory list of protocols for which the
manpage is relevant.

Most man pages already has a "PROTOCOLS" section, but this introduces a
stricter way to specify the relevant protocols.

cd2nroff verifies that at least one protocol is mentioned (which can be
`*`).

This information is not used just yet, but A) the PROTOCOLS section can
now instead get generated and get a unified wording across all manpages
and B) this allows us to more reliably filter/search for protocol
specific manpages/options.

Closes #13166

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curldown: Fix email address in Copyright

The curldown conversion accidentally replaced daniel@haxx.se with
just daniel.se. This reverts back to the proper email address in
the curld

curldown: Fix email address in Copyright

The curldown conversion accidentally replaced daniel@haxx.se with
just daniel.se. This reverts back to the proper email address in
the curldown docs as well as in a few other stray places where it
was incorrect (while unrelated to curldown).

Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Closes: #12997

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docs: make curldown do angle brackets like markdown

Make sure we use \< and \> in markdown all over so that it renders
correctly, on GitHub and elsewhere. cd2nroff now outputs a warning

docs: make curldown do angle brackets like markdown

Make sure we use \< and \> in markdown all over so that it renders
correctly, on GitHub and elsewhere. cd2nroff now outputs a warning if it
finds an unescaled angle bracket.

Ref: #12854
Closes #12869

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docs: remove spurious ampersands from markdown

They were leftovers from the nroff conversion.

Follow-up to eefcc1bda4bccd800f5a5

Closes #12800

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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