relocated admin integration into unique top level rst file.

[RobTilton]

Problem

Advanced_usage.rst was becoming massive.

Solution

copy paste section into a new file.

Acceptance Criteria

changelog.rst updated.

[matthewhegarty]

Thanks for picking this up. A couple of things left to do

1. Update index.rst so that the link to 'admin integration' appears after 'advanced usage'
2. There are still some sections which need to be copied over from advanced_usage, i.e from 'importing' onwards

Existing v4 docs are here for reference.

You can build the docs locally to check they look ok before pushing.

[matthewhegarty]

This is great - thanks