GitHub Actions: Check URLs using lychee

[Calinou]

This makes it possible to be aware of dead links and replace them with other resources (such as archived versions) as soon as possible.

Note that class reference URLs not checked within this repository, as I plan to also integrate lychee into godotengine/godot's CI (which has the source of the class reference files).

Note

CI will fail until we resolve #10674, so that we remove the download.tuxfamily.org links from the docs.

[tetrapod00]

Approve the idea. Shader function replacement links are correct.

Can't review the lychee action implementation

[skyace65]

I rebased this PR since tetrapod's PR was merged and now the URL check is failing on a different link. I'm not sure why though because the link works for me.

[Calinou]

I rebased this PR since tetrapod's PR was merged and now the URL check is failing on a different link. I'm not sure why though because the link works for me.

I've added gamedevartisan.com to exclusions. Some websites tend to block link checkers and will return various error codes (typically 429, but sometimes 403 or even 503 in this case).

[skyace65]

@mhilbrunner I'd like your review before merging this since it's adding another check

[mhilbrunner]

I think something like this is a good idea, and I didn't spot anything in the workflow. Looks good to me!

Some things can be expanded, but I think we want to see this in action first & iterate. So, lets go ahead.

[mhilbrunner]

Thank you!