Tagging

[pirog]

The old docs allowed for content tagging using frontmatter. It would be cool for the new docs to do this. Specifically i think we would want

• A composable called useTag() that would return similar information to useCollection(). You should be able to invoke it with no args to get all tags, a string to return a single tag, an array of multiple tags (and/or multiple string args) to get many tags.
• Page components similar to the collection ones so users can easily make Tag pages like this
• I think its probably easiest for this to be restricted to "collections" content but if it works for all content that would be cool too

Note that it probably easiest to augment some of the existing collections components to handle tagging vs adding more, mostly the same, ones.