refactor: general cleanup of internals and documentation

[avik-pal]

Main Changes

• similar to other changes in LuxDL packages, code is organized in submodules
• uses public to specify that is public and what isn't
  • [x] Layers
  • [x] Vision
  • [x] Basis
• @compact layers are great but are mostly meant to be used in research codebases (see recent discussion in NeuralOperators.jl)

Documentation

Documentation hosting going forward will be:

1. If Lux depends on the package: documentation will be hosted on the central docs.
2. If the package builds on top of Lux, it needs a dedicated docs site.

The reasoning is quite simple: currently making a breaking change in Lux creates a deadlock in docs building since Boltz needs to be updated before we can build the docs for lux and we can't release lux till docs build (since we have actual tests inside lux).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.75182% with 25 lines in your changes missing coverage. Please review.

Project coverage is 88.79%. Comparing base (6a97386) to head (5600531). Report is 15 commits behind head on main.

Files	Patch %	Lines
src/utils.jl	55.88%	15 Missing :warning:
ext/BoltzMetalheadExt.jl	86.66%	2 Missing :warning:
src/layers/hamiltonian.jl	80.00%	2 Missing :warning:
src/layers/mlp.jl	71.42%	2 Missing :warning:
src/patch.jl	50.00%	2 Missing :warning:
src/basis.jl	92.30%	1 Missing :warning:
src/vision/extensions.jl	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #47 +/- ## ========================================== - Coverage 91.84% 88.79% -3.06% ========================================== Files 22 22 Lines 319 339 +20 ========================================== + Hits 293 301 +8 - Misses 26 38 +12 ```

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