Add instructions for running mdbook linkcheck

[GeckoEidechse]

Allows checking if all relative links inside the mdBook are valid. Especially useful for ensuring updated links when moving content around.

[itscynxx]

Works and warns of links, I would suggest adding a section similar to:

mdBook-linkcheck

There's a community made package that you can use alongside mdBook to show you some potentially invalid hyperlinks in markdown files called mdBook-linkcheck

Although not required, you may find it helpful to use alongside mdBook to ensure you've linked things properly

To install mdBook-linkcheck use the following link if:

• you have Rust toolchain installed
• you DON'T have Rust toolchain installed (add mdBook-linkcheck.exe to the same folder mdBook.exe is located)

[GeckoEidechse]

Failing link is

https://help.ea.com/en/help/pc/link-ea-and-steam/

wtf?

https://github.com/R2Northstar/NorthstarWiki/actions/runs/6484396432/job/17608135877?pr=238#step:4:420

Need figure out if there's a way to ignore external links...

[itscynxx]

Need figure out if there's a way to ignore external links...

does adding follow-web-links = false under output.linkcheck change anything

[GeckoEidechse]

Need figure out if there's a way to ignore external links...

does adding follow-web-links = false under output.linkcheck change anything

Was about to try that until I realised that ocular-d/md-linkcheck-action@master uses https://github.com/tcort/markdown-link-check and not https://github.com/Michael-F-Bryan/mdbook-linkcheck (i.e. a completely different tool xD)

It seems there's no pre-made Action for mdBook-linkcheck :c

[GeckoEidechse]

I'm gonna skip CI for this PR and make a separate one instead ^^

[GeckoEidechse]

Gonna merge this as is for now and then focus on CI in a second PR