Add docs about logging within Agents

[BrianJKoopman]

Description

Describe the txaio logging and how to use it within Agents and supporting code.

Motivation and Context

After detailing how logging should be handled in a PR (simonsobs/socs#136) I figured I'd write it down in the docs.

How Has This Been Tested?

It's only documentation, but the practices described are used in several existing Agents.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Unless I am preparing a release, I have opened this PR onto the develop branch.