Document := operator better

[bvdmitri]

Fixes #359

This pull request includes an update to the docs/src/manuals/model-specification.md to clarify the correct syntax for creating deterministic relationships in RxInfer. The most important change is the addition of a new section explaining the use of the := operator for deterministic relationships.

Documentation update:

• docs/src/manuals/model-specification.md: Added a new section titled "Deterministic relationships" to explain the use of the := operator for creating deterministic relationships between (latent) variables, with an example provided.

[codecov[bot]]

Codecov Report

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

Project coverage is 84.99%. Comparing base (f5401c8) to head (83be7ff). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #365 +/- ## ======================================= Coverage 84.99% 84.99% ======================================= Files 20 20 Lines 1526 1526 ======================================= Hits 1297 1297 Misses 229 229 ```

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