Make sure that we handle relative link correctly

[romanowski]

Add better warning message in case of missing link.

We still are missing some links:

[WARN] /home/krzysiek/workspace/dotty/scala3doc/scala3-docs/index.html: Unable to resolve link 'blog/index.html' <- this is false positive
[WARN] /home/krzysiek/workspace/dotty/scala3doc/scala3-docs/index.html: Unable to resolve link 'docs/index.html' <- this is false positive
[WARN] /home/krzysiek/workspace/dotty/scala3doc/scala3-docs/docs/release-notes/0.1.2.md: Unable to resolve link '../reference/contextual/implicit-function-types.html'
[WARN] /home/krzysiek/workspace/dotty/scala3doc/scala3-docs/docs/reference/features-classification.md: Unable to resolve link 'contextual/delegates.md'
[WARN] /home/krzysiek/workspace/dotty/scala3doc/scala3-docs/docs/reference/features-classification.md: Unable to resolve link 'contextual/given-clauses.md'

@b-studios we probably should sort out that paths during documentation migration

[romanowski]

fixes #10465

[b-studios]

@romanowski agreed, we could restructure the content to also solve the link issues at once.

[abgruszecki]

LGTM!