Update constraints macro documentation

[wouterwln]

This is a first version for a rewrite of the constraints macro documentation. There's a couple of things that should happen before we merge this:

• [ ] Release RxInfer 3.0, half of the code in this example is written for the next GraphPPL version
• [ ] Decide whether to use DocumenterCitations. I kinda like the $\LaTeX{}$-like referencing, but it might be difficult to change this everywhere.
• [ ] Change the link for the BFE explanation to the actual page (when it exists)
• [ ] Change all julia tagged code blocks to @example manual_constraints
• [ ] Change all RxInfer 2.0 code to 3.0 code.

The explanation is on the short side now, I feel. Let's discuss on which topics we want to elaborate further and which we want to leave as short as they are now

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (cdadfb7) 81.03% compared to head (ebdb1ed) 81.03%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #172 +/- ## ======================================= Coverage 81.03% 81.03% ======================================= Files 11 11 Lines 1271 1271 ======================================= Hits 1030 1030 Misses 241 241 ```

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