This commits makes the following high-level changes:
* 'ShutdownHandle' is renamed to 'Shutdown'.
* 'Rocket::shutdown_handle()' is renamed to 'Rocket::shutdown()'.
* '#[launch]` is preferred to '#[rocket::launch]'.
* Various docs phrasings are improved.
* Fixed various broken links in docs.
This commits rearranges top-level exports as follows:
* 'shutdown' module is no longer exported.
* 'Shutdown' is exported from the crate root.
* 'Outcome' is not longer exported from the root.
* 'Handler', 'ErrorHandler' are no longer exported from the root.
The attribute is applied everywhere it can be across the codebase and is
the newly preferred method for launching an application. This commit
also makes '#[rocket::main]` stricter by warning when it is applied to
functions other than 'main'.
