Add Header Note for Previous Doc Versions

[mpstefan]

As a user of NGF on a version before 2.0
I want a header on the current docs to explain that I can go to GitHub to get docs for my version of NGF
So that I don't use docs with the 2.0 architecture for my older installation of NGF

Acceptance

• Add header note to all docs for pre-2.0 users to access their docs in the Github repo (since we don't have versioned docs published)

[ADubhlaoich]

Completed as part of 2.0 doc release changes.