ISIS-1810: updates with first draft of 2.0.0-M1 release.

* Replaced content/versions/SNAPSHOT with doc from "maint-2.0.0-M1" branch.
* Copied content/versions/SNAPSHOT to content/versions/2.0.0-M1

Andi had reworked the documentation.adoc file; because I wanted to make this go live I "backported" this:
* Copied documention.html over to content/documentation.html and content/current/documentation.html  (nb: already copied documentation.adoc over similarly to maint-1.16.2 branch in main isis.git repo)

Also:
* copied migration-notes.html and release-notes.html from content/versions/2.0.0-M1 to content/versions/current and content/*
  - to make it easier to discover
* manually patched the version in the html header.