New Scaladoc static site gen - Add a mention to the provided Liquid variables

[JD557]

I think it's a bit odd that the documentation specifies "[...] several custom filters and tags specific to Scala[...]" and then doesn't mention anything about that.

From what I can tell from the source code, the only Scala specific things being provided are the projectTitle and projectVersion.

I think it's useful to document the existence of this variables. Especially projectVersion, which can be used to automatically generate sbt/mill/maven/... coordinates.

[bishabosha]

@pikinier20 perhaps you know more about which liquid variables are available in the new scaladoc

[pikinier20]

Tbh I don't know. I guess it's some standard set. FYI we are using Jekyll flavor: https://github.com/lampepfl/dotty/blob/cf4dd05433c4d3866e86a55063791b57f7a5e278/scaladoc/src/dotty/tools/scaladoc/site/templates.scala#L116 .