docs: add PR template reminder, Makefile, and assets folder

[carlydf]

What changed?

• Add PR template reminder to update docs
• Add first d2 diagram to Matching README (more to follow)
• Refactor docs folder to contain an assets folder where we store images for all the READMEs in the project, instead of using github assets

Why?

To improve how we interact with and update docs

How did you test it?

n/a

Potential risks

n/a

Is hotfix candidate?

no

[stephanos]

Note to reviewers: I believe d2 is a useful diagram tool that has some notable advantages over Mermaid: it looks gorgeous and it has more options to get the layout you want (specifically, grid layouts). The downside is that it requires an extra rendering step since it's not natively supported like Mermaid. But in my experience, the diagrams you can create in d2 wouldn't even work in Mermaid - you'd have to draw them manually. So d2 enables us to generate more complex diagrams.

If it turns out d2 doesn't work out for us, we can still use the SVGs or re-create the drawings by hand, where needed. The hardest part of creating a diagram is figuring out the content, not rendering it. So the risk seems low.