ci: check dead links and broken file references

[novusnota]

With careful separation of internal documentation and public Tact docs. And introduction of a scheduled workflow (once a week) that checks external (https) links and auto-creates issues when such checks fail.

Also, resolved issues with some links. For example, the Certik blogpost gives 403, i.e. automatic link scans are probably detected and prohibited — and thus it had to be added to exceptions. Since the CI args in the public docs case are already rather long, I've decided to make a .lycheeignore file and put that blogpost there.

Issue

Closes #755.

Checklist

• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings

[verytactical]

In my opinion, CI shouldn't check external links. Third party services are often going down, and we'll have to fix every link before we'll be able to merge any PRs.

I'd rather keep it to only internal links, and create an optional or regular job for external links.