Add initial mdoc support

[regadas]

• mdoc compile is tied to Compile / paradox.
• README.md is now a symlink to index.md

[johnynek]

Thanks! We would need to also run mdoc in the CI no? or is it automatically done by doc? Did you very the CI fails if you break some code?

[regadas]

@johnynek I was trying to tie it to the compile task but it seems to be a little bit tricky... the easy way was to add docs/mdoc as one of the steps. CI should fail if code breaks.

We will need to limit the docs build to 2.12/2.13 though ... I'm seeing some issues with dotty.

[regadas]

@johnynek added a separate step for docs ... ok tried ... I need to see how to include the setup steps.

[johnynek]

thanks @regadas ! I don't really know much about the sbt plugin we are using to generate the yml, but it seems like it is doing the right thing.

Thanks.

[johnynek]

Related to #7