Document process of deploying with a separately managed repo of customisation resources

[rob-metalinkage]

Customisations should be managed separately from the codebase, so it will need to be clear how customisations are deployed against the codebase.

[nicholascar]

They are now, and the process for a particular installation's customisation is documented, per installation, in the overlay repository. The OGC's overlays are applied using the file apply.py in the OGC theming repo: https://github.com/surroundaustralia/VocPrez-theme-ogc.

Still could do with a Customisation section in the main documentation though.