Refactor HTTP Files and add Image Generation

[martinpeck]

This PR refactors the .http files so that...

• one file per API (to reduce clutter/length of files)
• add "REST Client environments" so that its easy to point the HTTP files at either local or azure
• add new image gen calls to the suite of HTTP files, and the required env var to support this

Closes #47

[martinpeck]

There's some overlap in what I've changed here, and what @hannesne has updates/change in the docs related to "getting started".

• If this gets merged before Updated docs a bit to help guide through first run experience. #46 then Updated docs a bit to help guide through first run experience. #46 needs tweaking.
• OR, if Updated docs a bit to help guide through first run experience. #46 gets merged first (which might be easier) I can tweak the docs to reflect the new location/file names, plus make sure that the instructions including choosing the REST Client environment.

So...my suggestion is merge #46 first, and assume any approval of this PR includes fixing the docs up first.

[martinpeck]

This would close #47

[stuartleeks]

LGTM - a couple of minor typos.

This is definitely cleaner than the previous approach - thanks