Fix Lychee URL checking on CI

[Calinou]

This also adds the cache file to .gitignore in case you run Lychee locally with the same options as CI.

There is still one error I'm trying to fix:

Issues found in 1 input. Find details below.

[engine_details/architecture/object_class.rst]:
 [ERROR] error: (at 397:3) | Cannot parse 'res://someresource.res::1' into a URL: invalid port number

Adding res://someresource.res::1 to .lycheeignore or passing --exclude res://someresource.res::1 didn't help.

[mhilbrunner]

LGTM, thank you for taking the time to fix these!