Development: Add a page on extensions

[duncandewhurst]

There was a to-do in #27 to add a section on extensions and extensibility to the data modelling and schema development page. However, I'm not sure extensions fit under that page exactly, and they are a large enough topic to warrant a page of their own.

The building an extensions mechanism for BODS concept note could be used as a source for content.

As part of preparing the new page, we should check if any patterns and components need to be added or updated.