Document better the purpose of the Dotty website

[julienrf]

Currently, the role of the Dotty website is not clear. Its content seems to overlap with the Scala documentation website.

I believe the Dotty website should be used to host the “nightly” documentation only (as opposed to “stable” documentation), and internal documentation for developers of the compiler (see related issues #15110, #15111, and #15113).

• We should make this role clearer by adding a message about it on the front page, with a link to the stable documentation (https://docs.scala-lang.org).
• We should add a banner to the top of every page to highlight the fact that the content reflects what is in the nightly state, with a link to the stable documentation (similar to the movedTo property).

[julienrf]

I see that currently https://dotty.epfl.ch does not show anymore the information “This is a nightly documentation. The content of this page may not be consistent with the current stable version of language.” It seems that there is a regression and the work done in #15117 has been lost.