Add link-checking workflow for docs

[aljazs-flare]

Internal Jira ticket id: FLR-69

[segfaultxavi]

I'm afraid this solution does not work. You can see this PR is already failing the link check.

I can see at least two problems:

• We use a markdown extension that automatically searches for files when they don't have a path. So, for example, ./file.md will look for the file in the same folder, but file.md will trigger this extension, locate the file anywhere in the tree and produce the correct link.
• We use another markdown extension that allows us to create anchors, but the link checker does not recognize them.

I don't think any link checker that parses the md files will be able to fix this. Maybe there are solutions that parse the generated HTML files instead?

[aljazs-flare]

• We use a markdown extension that automatically searches for files when they don't have a path. So, for example, ./file.md will look for the file in the same folder, but file.md will trigger this extension, locate the file anywhere in the tree and produce the correct link.

from my point of view this is sort of a hack. Just use relative paths from the get-go. I can include the patch for this in this MR.

• We use another markdown extension that allows us to create anchors, but the link checker does not recognize them.

Anchors are fully supported by markdown. Is this extension really needed?

I don't think any link checker that parses the md files will be able to fix this. Maybe there are solutions that parse the generated HTML files instead?

I think that we should be validating the source files, not artifacts (generated HTML files).