Agnostic_baker: Improve existing documentation

What

Parent MR: !16492 (merged)

Why

For easier understanding of the product, better testing and better reviewing process.

How

• improving existing inline documentation of src/bin_agnostic_baker library
• adding a README document
• adding a CHANGES changelog entry

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR