## Problem
The current stripe size of 256 MB is a bit large, and can cause load
imbalances across shards. A stripe size of 16 MB appears more reasonable
to avoid hotspots, although we don't see evidence of this in benchmarks.
Resolves https://github.com/neondatabase/cloud/issues/25634.
Touches https://github.com/neondatabase/cloud/issues/21870.
## Summary of changes
* Change the default stripe size to 16 MB.
* Remove `ShardParameters::DEFAULT_STRIPE_SIZE`, and only use
`pageserver_api::shard::DEFAULT_STRIPE_SIZE`.
* Update a bunch of tests that assumed a certain stripe size.
We want to switch away from and deprecate the `--compute-hook-url` param
for the storcon in favour of `--control-plane-url` because it allows us
to construct urls with `notify-safekeepers`.
This PR switches the pytests and neon_local from a
`control_plane_compute_hook_api` to a new param named
`control_plane_hooks_api` which is supposed to point to the parent of
the `notify-attach` URL.
We still support reading the old url from disk to not be too disruptive
with existing deployments, but we just ignore it.
Also add docs for the `notify-safekeepers` upcall API.
Follow-up of #11173
Part of https://github.com/neondatabase/neon/issues/11163
Not sure if this should actually be a link pointing to the
`persistence.rs` file but following the conventions of the rest of the
file, change `persistence.rs` reference to simply be a file name
mention.
## Problem
External contributors need information on how to use the storage
controller.
## Summary of changes
- Background content on what the storage controller is.
- Deployment information on how to use it.
This is not super-detailed, but should be enough for a well motivated
third party to get started, with an occasional peek at the code.
