GHA: spellcheck

GHA: spellcheck

This spellchecker checks markdown files. For this reason this job
converts all man pages in the repository to markdown with pandoc before
the check runs.

The perl script 'cleanspell' filters out details from the man page in
the process, to avoid the spellchecker trying to spellcheck things it
can't. Like curl specific symbols and the SYNOPSIS and EXAMPLE sections
of libcurl man pages.

The spell checker does not check words in sections that are within pre,
strong and em tags.

'spellcheck.words' is a custom word list with additional accepted words.

Closes #9523