Clarify customisation support for Sphinx Stack

Our Sphinx Stack is intended to provide a solid, featureful experience out of the box, including several high value extensions and customisations. However, Sphinx itself is intended to be as extensible as a user would like - and our capacity to support additional "non-standard" customisations is limited.

It's desirable to create a topic in the documentation outlining our approach to supporting issues that arise with use of the Sphinx Stack. While we should prioritise and assist with any issues stemming from default implementation of the tooling, we should clarify that issues arising from additional customisation and extension of Sphinx beyond this set of supported tools will not be a priority.

To clarify, we will support users as much as we can - but there's a [huge world out there](https://pypi.org/search/?c=Framework+%3A%3A+Sphinx+%3A%3A+Extension) of Sphinx extensions (not including custom scripting) that will take time and resources to support - and we may not have the capacity.