Improve maintainability of common elements

[gordjw]

The static version of this site is great from a simple maintenance point of view.

However, making change to common elements (header, navigation, footer, metadata, etc) is quite difficult. For each change, almost 800 files must be edited.

To fix this, we could introduce a simple template system that allows common elements to be defined once and reused.

Acceptance criteria

• Changes to common elements like the header can be done by editing as few files as possible
• Editing and deployment are not significantly more complex or difficult