Add documentation sidebar category labels

[caendesilva]

In the docs rewrite I'm adding categories to the front matter, thinking they can be used to label and group together items in the sidebar.

---
category: "Digging Deeper"
---

Wondering about the best implementation. I have an idea that may not be very efficient but could "work for now":

Scanning all the documentation page files, assembling a list of all the categories used then matching them with the priorities to the label can be injected before the first post in the category.

[caendesilva]

Re: implementation:

Alternatively, each category could reside in its own <ul>. This may be the better (and more semantic and accessible) option.
Maybe to keep the markup light the feature is enabled if at least one markdown page has the category front matter. Any pages that does not have a category gets grouped in "other"