Gaussian-Gaussian conjugate prior (#43)

[affeldt-aist]

Motivation for this change

• add Gaussian-Gaussian conjugate prior

Developed with the assistance of Claude Opus 4.8 and Fable 5 (1M context).

fyi: @gbdrt

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

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[affeldt-aist]

I made a few final simplifications and I think that the code is ok but
the documentation can be improved. A few points:

What does "complete the square" (two occurrences) mean?

The comment before the lemma normal_pdf_conjugate is talking
about conditional probability but this does not appear
explicitly in the statement.

The comment before the lemma normal_prob_conjugate is talking
about theta but this is not a free variable in the conclusion.

The lemma normal_prob_conjugate is supposed to correspond to the first row of the table
https://en.wikipedia.org/wiki/Conjugate_prior#When_the_likelihood_function_is_a_continuous_distribution

This could be mentioned in the documentation and used to make the documentation more precise by using
similar vocabulary.

@gbdrt ?