Update documentation for fast track availability request

[hxtomprice]

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

Updates the documentation for the fast track availability request to include a hard coded list of locations for this product.
This is causing some confusion for our partners as the documentation tells partners to call the location endpoint which does not support requests for a fast track product.
The list of fast track locations is fairly static, so hard coding this for now looks like a sensible suggestion for the time being

Any tech debt?

Hard coded lists of locations isn't ideal, we should revisit this when we have a consistent data source for this

Screenshots / Screencast

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[hxtomprice]

I'll rebase this and fix the conflicts once #226 is through

[david-r-edgar]

approving, but still needs updating