Add user guide on how to extend Beluga

[hidmic]

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)
• 📚 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.

• 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
• I have added necessary documentation (if appropriate)
• All commits have been signed for DCO

[hidmic]

As per #306, it looks like I'm missing a custom sampling view. I'll add that next.

[nahueespinosa]

@hidmic Looks great!

[hidmic]

As per #306, it looks like I'm missing a custom sampling view. I'll add that next.

I think I'm going to hold this off until we have more resampling algorithms. We only define a naive multinomial sampler. There's #48, there's stratified resampling, there's residual resampling, and more. I presume we'll be using views all the way, but some algorithms have to perform multiple passes and carry additional state so the standard resampling algorithm structure may end being a tiny bit more complicated than beluga::views::sample.

[nahueespinosa]

LGTM!