Document directory layout

[norswap]

Document the directory layout used by the repo.

This includes both directories we created ourselves + shenanigans done by the Optimism monorepo (cf. paths.py for details on that).

We could also consider moving some of the stuff created by operations in the monorepo out of it for better discoverability (having to browser to clones/optimism/packages/bedrock-contracts/deployments gets old pretty quickly).

Note that we can't manage these files entirely out of the monorepo because some of these paths are hardcoded in the logic, this is most notably the case in the deploy scripts (Deploy.s.sol etc).

This probably should be done as the last task of the big-refactor milestone.