V2 beta docs

[hxtomprice]

What does this PR do? (please provide any background)

Adds documentation for the v2 beta version of the API.
This is an early design for a new version of the API. Schemas and integration guide are subject to change
This will be available under docs.holidayextras.co.uk/partner/v2/ but will not be discoverable from following any links until this is ready.

Any tech debt?

This is subject to change before use.

Screenshots / Screencast

• I have checked our general contributing document and the project specific contributing document (if present) and I'm happy for this to be reviewed.

By approving a review you are confirming you have...

• Witnessed the work behaving as expected (this could be on the author's machine or screencast).
• Checked for coding anti-patterns.
• Checked for appropriate test coverage.
• Checked all the tests are passing.

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