'Developer' documentation of this Google Apps Script in the growth.rcpch.ac.uk site

[pacharanero]

At the moment since there is not seemingly a large audience for this plugin, we can simply document how you might install this for development/testing purposes, ie this would be a more technical and a less straightforward workflow.

When we have it automatically being deployed to Google Workspace Marketplace, we can also document the steps for installing it from there, which would be the 'easy' and user-facing workflow when we have it ready for use in that way.

Note linked Issues on the doco site
rcpch/digital-growth-charts-documentation#13
rcpch/digital-growth-charts-documentation#7

The above can both be closed once we have something detailing this plugin on the doco site