Reorganise examples page (#2314)

Reorganise examples page

* some examples use multiple tables, in this case I have used what seems to be the most relevant table (e.g. get the first medication event for a patient with a valid registration => medication)
* add links in both directions between the examples and the table reference
* I think this examples page should contrast with e.g. the cheatsheet page by doing less of providing composable blocks (cheatsheet) & more concrete examples that could be adopted wholesale. Currently the examples page introduces reusable concepts across multiple tables in a slightly ad-hoc way - I think it should do less of this & instead provide a specific (and ideally useful) example for each table.