Add overview to new monorepo to "Developer" section of docs

[jkhartshorne]

Something that explains (or points to a tutorial on) changesets, etc.

Basically, if somebody had never seen a monorepo, didn't know about changesets, etc., what would they need to read to make sense of things.

Asking for a friend. :)

[jkhartshorne]

@jessestorbeck -- maybe assign this to someone?

[jessestorbeck]

Some of this is covered in the contributions section, but a "repo orientation" section might be nice too and have some additional information, e.g. Q: "What does pyproject.toml do?"; A: "We use the Python package manager Poetry in conjunction with MkDocs to build our documentation; pyproject.toml tells Poetry how to set up the build environment for the docs."

@hunterschep @ayang21 -- This seems natural to include in Getting started on development. Maybe as the second #-level heading?

[jessestorbeck]

I've take a stab at this in 67d101579d479df7fc1cbb2793bdf3802cb53420. It's folded into working on the linting setup though, so not currently ready for a PR. It looks like this though:

And with the annotation expanded:

I went back and forth about what was the best way to format it. I tried a diagram with Mermaid (like the contribution flowchart) and that was terrible. Annotations seemed like the best way not to overwhelm the page with text.