[dev docs] Show different retry methods available, and when to use each

[miketheman]

There's a few different retry mechanisms in the codebase, and each has its own approach/gotchas that should be detailed for the readers.

A few approaches used in warehouse:

• pyramid_retry and warehouse config
• Celery built-in, retry()
• Celery built-in, @task(autoretry_for=...

Other things to note, might be out of scope:

• request.http sets max_retries to 1 and doesn't appear to be configurable (yet)
• There's @google.api_core.retry decorators in currently-unused GCSFileStorage/GCSSimpleStorage classes, might be helpful to note that when using a service's SDKs, use their exceptions/retries? Uncertain.
• Response.retry_after - sets a header to inform clients on when to retry