Automate Components index page

Problem: We have an index page for our components, but right now it must be manually updated and is difficult to keep in sync

Goal: Automate the list of components + descriptions that show up on primer.style/design/components

Why this matters: Though you can't click on Components from the navigation, there will likely be many places where we will want to link to this 'landing' page from, such as the main Primer homepage.

Exit criteria:

• List of components that appear on https://primer.style/design/components matches the list of components in our documentation that appear in the navigation.
• Each component should have name and description (thumbnails are nice to have, but not necessary for v1)