Add codec categories in doc contents

[dstansby]

This sorts the existing codecs into three sections within the docs table of contents: Compression, Filters, and Other. I often find it hard to distinguish between these as the docs are currently, so I'm hoping this makes the docs a bit easier to read.

This also greatly improves the contents structure of the docs, giving three top-level pages:

1. API reference
2. Release notes
3. Contributing guide

Here's what the new API reference page looks like: https://numcodecs--458.org.readthedocs.build/en/458/api.html

I'm definitely not an expert in codecs, so I may have got some of the categories wrong!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (a8f6efb) to head (38e8f72).
Report is 69 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #458   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          59       59           
  Lines        2334     2334           
=======================================
  Hits         2332     2332           
  Misses          2        2           

[dstansby]

ping @zarrdevelopers, any chance someone could review this?

[dstansby]

Since this has been open for a year, and it's a month since I pinged for reviews I'm minded to merge it without a review since it's a big improvement to the docs and doesn't change any functionality. Unless anyone shouts in the next two weeks I will merge this.

[d-v-b]

thanks @dstansby!