Introduce Bethe Free Energy and variational inference concept in the documentation

[bvdmitri]

The current documentation for RxInfer lacks clarity regarding the concept of Bethe Free Energy and does not sufficiently address the importance of working with it. Additionally, the documentation does not provide comprehensive information about the concept of variational inference. The existing documentation page dedicated to Bethe Free Energy fails to provide a straightforward explanation of its essence.

We require an introductory section in the documentation that offers simple, plain-language explanations for the following concepts:

• Variational inference.
• Variational Free Energy.
• Bethe Free Energy constraints.

• This addition will enhance the documentation's accessibility and clarity for users.

[bvdmitri]

This task has been added to the milestone for tracking and prioritization.

[bvdmitri]

@ThijsvdLaar This task has been added to the milestone for tracking and prioritization.

[bvdmitri]

Linked issue, can be addressed together https://github.com/biaslab/RxInfer.jl/issues/95