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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cmdline-opts/_EXITCODES: sync with libcurl-errors

- Add error code 100 (CURLE_TOO_LARGE) to the list of error codes that
can be returned by the curl tool.

Closes https://githu

cmdline-opts/_EXITCODES: sync with libcurl-errors

- Add error code 100 (CURLE_TOO_LARGE) to the list of error codes that
can be returned by the curl tool.

Closes https://github.com/curl/curl/pull/13015

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docs: more language cleanups

- present tense
- avoid bad words

Closes #13003

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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