Document how files are stored and what happens when they are removed

[miketheman]

Probably a good article for docs.pypi.org General need is to describe:

• what data gets stored where (metadata in warehouse, actual files reside in storage buckets, downloads exist in BQ)
• what happens when a project/release/file gets removed - which data changes (simple index is gone

[woodruffw]

Triage: this is probably a good fit for the dev-docs, right?