Documentation fixes

[danacotoran]

Fix deploy to Github Pages (hopefully)

The deploy to GH Pages action is failing since adding the documentation on how to install the header with the Prototype Kit.

This is because GH Pages uses Jekyll to convert .md into .html, which uses Liquid as a templating language, which has a very similar syntax to Nunjucks.

The snippets of Nunjucks language in our documentation are picked up as Liquid code, causing errors in the deploy to Pages, as Liquid does not have the same syntax as Nunjucks so certain keywords such as set are not recognised.

This replaces some of the Nunjucks syntax in our documentation (the {,} and % characters) with HTML entities which should not be recognised as Liquid code. It also swaps the Markdown syntax for code snippets with actual HTML tags, as it appears HTML entities don't render correctly inside `...` and ```...``` blocks.

(Attempt to) fix image paths in GH Pages

Change image paths on our documentation to relative paths, which should be picked up by Github, and Github Pages too. Currently they work in the code preview in GH but not in the version deployed on Pages. Source: https://github.blog/2016-12-05-relative-links-for-github-pages/

[alex9smith]

I'll temporarily set Pages to deploy from this branch so I can check it looks ok