Use project scope for mdoc sources setting, not ThisBuild

[pdalpra]

This fixes an issue in sbt-microsites where mdoc would always look for sources in a docs folder at the root of the sbt build, instead of a looking for it in the module's base directory.

[pdalpra]

The build failure is linked to a test compilation error.
I opened #358 with a fix for it.

[juanpedromoreno]

Hi @pdalpra , thanks for your time. I've just merged #359, could you please rebase against master? Thanks!

[pdalpra]

No problem, I just rebased this PR on master and closed #358, obsoleted by the revert

[pdalpra]

Forgot to mention, but I tested this PR locally by overriding mdocIn in my build.

It seems however than mdoc leaves many warnings as it detects links to other pages that are not yet built as 'unknown', but I don't know mdoc well enough to see if there is something to do about it or if I misconfigured it 🤔

[pdalpra]

After checking mdoc documentation, these warnings can be avoided by disabling link hygiene:

mdocExtraArguments += "--no-link-hygiene"

[juanpedromoreno]

Thanks @pdalpra !