Update the documentation for the initialization syntax + minor fixes

[bvdmitri]

This PR makes the documentation for RxInfer better by:

• Moving important details about initializing nested models to the main guide. The migration guide now points to this main guide.
• Adding more info about how RxInfer handles missing values.
• Making some small fixes.

Fixes #298

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.19%. Comparing base (646ca25) to head (ba76599).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #303   +/-   ##
=======================================
  Coverage   83.19%   83.19%           
=======================================
  Files          18       18           
  Lines        1297     1297           
=======================================
  Hits         1079     1079           
  Misses        218      218           

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