Per T1231, a dichotomic search highlighted the last nightly toolchain able to
build Rocket v0.4 is nightly-2024-12-15. This change adopts that version to
generate documentation through Jenkins pipeline and documents it in README.
As we want to support more recent frameworks, a modular structure is required:
- different crates by framework can be provided
- common patterns can go to a core crate
To adopt this repository structure, the current code is moved to rocket-legacy/
In next commits, framework-agnostic code will be moved to the core crate.
Ref T2131 - required toolchain version
Ref T2135 - repository structure