Updated documentation to describe how to select for export in Admin UI

[matthewhegarty]

Problem

It is not clear from the documentation how to export selected items (e.g. StackOverflow question).

Solution

I have updated the documentation to describe how to do this.

Also bumped pre-commit version and fixed E231 errors (CI build was failing).

Acceptance Criteria

built docs and tested locally.

[coveralls]

coverage: 100.0%. remained the same when pulling edd674c on matthewhegarty:select-for-export-doc-update into 9dee0b3 on django-import-export:main.