Doc merge up

[tacaswell]

PR Summary

Starting to merge the whats_new and api_changes files up into the main ones. These still need to be edited and grouped better.

[dstansby]

@tacaswell I'm happy to give editing a shot when this is merged.

[dstansby]

I trust that everything has been copied across fine. Maybe in future we should get contributors to just add a section to api_changes.rst, or make api_changes.rst automatically include all the individual files to avoid having to do all this copying across.

[tacaswell]

We have people put things in separate files to prevent persistent merge conflicts as everyone tries to edit the same text file!

For the dev docs we do automatically include them, but it seem better to copy them all into one file and then do a final edit on them (so we can have opinions about order and merge / split entries as needed) for the final release.

Going to self merge on the strength of @dstansby 's approval and to avoid collecting anymore merge conflicts.

[tacaswell]

and the circle CI jobs passed.