Document default-features of satellite serverless functions

[peterpeterparker]

When a dev eject the satellite, the lib.rs is generated with all hooks. The developer can, if they wish, remove all hooks by setting default-features to false

junobuild-satellite = { path = "../libs/satellite", default-features = false }

It's also possible to speficy only a subset of the features

default = ["on_set_doc", "on_set_many_docs", "on_delete_doc", "on_delete_many_docs", "on_upload_asset", "on_delete_asset", "on_delete_many_assets", "assert_set_doc", "assert_delete_doc", "assert_upload_asset", "assert_delete_asset"]

We should document that.

For example

junobuild-satellite = { version = "x.y.z", default-features = false, features = ["assert_set_doc", "assert_delete_doc"] }