Set up v6.1.x versioned documentation as default documentation

[JoelSpeed]

Description

Sets up a 6.1.x documentation set and makes this the default displayed.
Users can access master branch documentation using a dropdown in the header.

I chose 6.1.x since this documentation should z releases to an x.y.z version, I think this is a format we should keep going forward (eg 7.0.x will be the next version and cover 7.0.0 and any 7.0.z we create.

Motivation and Context

Allows us to keep versioned documentation for the latest release without exposing to users that there are new features being worked on. We will get fewer issues raised saying a documented issue isn't in a given release

Fixes #550

How Has This Been Tested?

yarn start in the docs repo to view changes locally

Checklist:

• My change requires a change to the documentation or CHANGELOG.
• I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.

[NickMeves]

I can't remember, were we thinking of making the Docs its own repo?

Its always been a little annoying when we have docs merges, that other code PRs potentially have to rebase and get re-approved before merging.

[JoelSpeed]

I had thought we had agreed to keep them in the same repo since having them separate would mean that users who introduce new features would have to open two PRs rather than one, so it increases the barrier for entry for contribution