Finish starlight setup

[LauraLangdon]

Removing the Astro/Starlight content from the templates and making the docs our own.

[LauraLangdon]

Ready for review! The process I think would be optimal is:

1. Use the new setup instructions in the docs readme (here in the pr at docs/readme.md) so you can serve the docs site locally (and proof the instructions along the way!)

2. Have a look through and see if anything jumps out at you as being an issue with the docs site itself, not the content of the docs themselves (file issues for anything you notice wrong with the docs themselves!). I didn't make any changes to the docs content except to fix formatting/typos.

NB: the index.md/landing page for each section is mirrored from the README.md of that section's own repo, and after this PR merges I'll work with folks to set up a script or whatever to trigger a PR in the docs repo whenever a file with a mirror is updated, so the mirror in the docs is updated as well.

Let me know if ^^^ doesn't make sense!

[ll-zerr]

Question: I followed the instructions in the README.md to get the site running locally, but the sites that appear are the Starlight template ones. Am I doing something wrong in the setup?

yarn dev takes me to the "Welcome to Starlight" template on localhost4321
yarn build then yarn preview takes me to here: "https://starlight.astro.build/" on localhost4322

I couldn't find how to access our docs in there anywhere to preview.

[LauraLangdon]

Question: I followed the instructions in the README.md to get the site running locally, but the sites that appear are the Starlight template ones. Am I doing something wrong in the setup?

yarn dev takes me to the "Welcome to Starlight" template on localhost4321 yarn build then yarn preview takes me to here: "https://starlight.astro.build/" on localhost4322

I couldn't find how to access our docs in there anywhere to preview.

Had you checked out my branch? I think I said something less helpful like “in this PR”, sorry, but you'd need to run git checkout finish-starlight-setup in your local clone of the docs repo (there's a handy copy button next to the branch name!).