Merge branch 'mr-docs' into 'main'

[codebase](https://gitlab.torproject.org/tpo/core/arti/), or getting to

know our [architecture](./doc/dev/Architecture.md).

Before you begin working on a large feature or bug fix,

you may want to ask other Arti developers whether it's a good topic to contribute to.

You can do this by opening a gitlab ticket (or commenting on an existing ticket),

or asking on the [`\#tor-dev` IRC channel on OFTC](https://www.torproject.org/contact/).

Someone else may already be working on this topic,

or if it's related to an open ticket the ticket may be out of date.

This helps to avoid situations where you've spent a lot of time on an MR,

but we need to close it since it conflicts or is adjacent with active work by someone else.

More tests would always be great. You can look at the [coverage reports](https://tpo.pages.torproject.net/core/arti/coverage/)

to find out what parts need the more love.