Provide a method for contributing HTML snippets / code

[occupant]

Problem
We need a way to accommodate contributed HTML, both in terms of documentation and outputting code / pages.

This HTML format should be a superset of HTML so as not to impede novice users (if they want to just write HTML, we shouldn't discourage that).

Should allow the use of includes / nested templates so that code can be isolated and reused.

Proposed Solution

Twig

Alternatives considered
Plain HTML - poor choice due to high maintenance cost, error prone
Markdown - poor choice due to limits of class / attribute additions
Pug / Jade - poor choice due to not being easy to use plain HTML

Additional context
Happy to consider any other alternatives - these are just the ones I'm most familiar with.

[hazuremon]

Team have reviewed multiple static site generator options (15+) and have identified Gatsby as potential candidate as the tool. Next step is to explore and prototype Gatsby in more detail as the go to tool.