A wide pass on docs
1. Added an autoencoder example to README to showcase modules with
multiple methods
2. Remove FAQ (it was all on the old API) -- we can consider bringing
it back but best to just link to GitHub discussions if possible?
3. Mark `flax.nn` as deprecated in more places and make it less visible
4. Add deprecation notice in examples/README. (I will later remove the
directory entirely but I wanted to separate those)
5. Small adjustments
