nav: add tags and cats

Move `gh-pages` branch files into `/docs` and add test files
- Jekyll ignore `/docs` and `/test` folders when using from root
- Update Staticman config to point to correct branch and data file location
- Replace `{{ base_path }}` references with `absolute_url` filter
- Update documentation

Merge branch 'release/4.0.4'

Conflicts:
	README.md

Merge branch 'feature/theme-gem' into develop

Add `_data` files to root for those forkers of the repo

Move image assets in `/assets/images`

Merge branch 'release/3.2.13'

Merge branch 'feature/data-files' into develop

Change navigation links to sample data

Merge branch 'release/3.0.0'

Merge branch 'feature/remove-sample-files' into develop

Reduce config to bare minimum and remove sample content

Merge branch 'feature/documentation-and-demo' into develop

Update docs

Update stylesheets and JavaScript docs

Update docs

Update docs

Update archive layout docs

Update layout docs

Update docs