Add a "disable-ffmpeg" feature

[Polochon-street]

Still need to change some things around the CUE module, and make sure everything makes sense the way it's split (maybe use static .npy files instead of decoding files in the chroma tests?)
Need to update the CI as well with a disabled ffmpeg test

[Polochon-street]

TODO:

• Write CHANGELOG.md [x]
• Fix the CI to make sure all use cases are covered (run test + examples + clippy for all of them, e.g. with library and ffmpeg, with library alone, etc) [x]
• Write documentation on how to switch from old way to new decoder way [x]
• Fix README.md, etc (making sure that the path to FFmpeg/Decoder is correct) [x]
• Make sure all documentation links work
• Fix bliss-python? [x]