Add user guide on how to extend Beluga (#450)

### Proposed changes

Closes #306 . This patch adds a guide on how to extend Beluga. I covered
the fraction of the library that is the most stable regarding patterns
and architecture and also the most likely one for a user to extend --
new kinds of particles filters or support abstractions I skipped.

#### Type of change

- [ ] 🐛 Bugfix (change which fixes an issue)
- [ ] 🚀 Feature (change which adds functionality)
- [x] 📚 Documentation (change which fixes or extends documentation)


### Checklist

_Put an `x` in the boxes that apply. This is simply a reminder of what
we will require before merging your code._

- [x] Lint and unit tests (if any) pass locally with my changes
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] I have added necessary documentation (if appropriate)
- [x] All commits have been signed for
[DCO](https://developercertificate.org/)

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Signed-off-by: Michel Hidalgo <[email protected]>