Auto branch docs

[pp-mo]

Closes #3396

NOTE: I have made version changes (2.2 --> 2.3) to the .RSTs in 3 places :

• docs/iris/src/_templates/index.html
• docs/iris/src/_templates/layout.html
• docs/iris/src/whatsnew/index.rst

In future, to make this scheme work properly (so branch docs are properly readable, especially the whats-new), we really need to move these on just after a new release, when we bump the version in master.
That is, exactly as we do to the iris version string.

[pp-mo]

Working as-is (at last!).
Resulting docs here : https://scitools-docs.github.io/iris/auto_branch_docs/index.html

-- it's there because this source branch is in SciTools/iris, not in my fork as usual.
( So, we need to delete that later... )

[lbdreyer]

Thanks @pp-mo!

It will be great to get the auto docs building back!
I have just a few comments ...

[pp-mo]

Thanks @lbdreyer , I hope I've addressed those points now.
Mostly : I have added a 'special' mode to the aggregation as suggested, and it produces a temporary whatsnew page with a temporary non-version name.
See : https://scitools-docs.github.io/iris/auto_branch_docs/whatsnew/index.html

[lbdreyer]

Looking good! Thanks @pp-mo !