Add eyepiece pages

[peytondmurray]

This PR adds two pages related to eyepieces:

1. Ernest's list of eyepieces, recreated here in a much nicer (sortable) table
2. The 2023 buyer's guide to eyepieces, recreated from CN.

In the future I'd like to have some graphs with some useful information, but it was taking me too long to do this so I thought I'd stop and get this merged before spending more time.

Changes

• Added openpyxl dependency, allowing us to ingest excel files
• Added .gitignore entries for some yarn files I needed for running eslint
• Parallelize sphinx build to go faster
• Added functions in conf.py to ingest the raw excel for the eyepiece buyer's guide and turn it into json for the site at build time
• Made the commit history table more robust (doesn't break the build if you've just added a new file and haven't committed it yet, but you want to build the site anyway)