Adding base for future Spec into the compiler repo (#16825)

This is a cleaned-up version of the Scala 2 Specification, designed to
be used as basis for the Scala 3 Specification

`docs/_spec/README.md` describes how to build locally, this uses the
same system as the Scala 2 Spec
It will probably be possible to switch to a Scaladoc solution later

I highly recommend you review commit by commit instead of file per file

Author: bishabosha

.gitignore

@@ -93,3 +93,5 @@ compiler/test-coursier/run/*.jar
 # docs related
 contributors.js
 content-contributors.css
+docs/_spec/_site/
+docs/_spec/.jekyll-metadata