Add Header Note for Previous Doc Versions

**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)