Specify public API and map shallow to deep imports

[Helveg]

Describe the work done

Everything can be imported from bsb. from bsb import Morphology

List which issues this resolves:

closes #390

Tasks

• [ ] Add type annotations and __dir__
• [x] Added tests
• [x] Updated documentation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.13858% with 58 lines in your changes are missing coverage. Please review.

Project coverage is 85.43%. Comparing base (35afc0c) to head (c9f0fa4).

Files	Patch %	Lines
bsb/__init__.py	87.05%	55 Missing :warning:
bsb/cli/commands/_projects.py	0.00%	2 Missing :warning:
bsb/config/refs.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #813 +/- ## ========================================== + Coverage 85.17% 85.43% +0.26% ========================================== Files 104 104 Lines 13409 13755 +346 ========================================== + Hits 11421 11752 +331 - Misses 1988 2003 +15 ```

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