Add Tags to the documentation site

[pacharanero]

Material for MkDocs has support for Tags which are applied in the YAML front matter of each page and are indexed into a [TAGS] section which we can render in a tags.md page.

This allows us to superimpose another layer of discoverability on the documentation site, which is now quite large. Users may find what they need via navigating the 'tree' of headings and content, they may find content via search, but they may also find from the tags menu all the parts of the documentation that is tagged of relevance to that subject.

https://squidfunk.github.io/mkdocs-material/setup/setting-up-tags/#adding-tags