ARCHITECTURE.md now includes module visibility documentation

[Tim Van der Lippe]

Hey everyone,

Since the folder restructuring and GN metadata efforts have concluded, we have now updated the ARCHITECTURE.md documentation with guidance on the dependency graph between modules. Please take a look at https://chromium.googlesource.com/devtools/devtools-frontend/+/master/ARCHITECTURE.md#folder-structure to see the ideal dependency graph between modules. Note that this is the ideal outcome, but currently does not match reality for several folders. Over time, we hope to adhere more and more of the older folders to these dependency rules.

Cheers,

Tim