Update Documentation

[Mattjet27]

What does this Pull Request accomplish?

Makes various updates to the documentation to be published via GitHub Pages:

• Fix typo in example of InstrumentAbstraction.md documentation: should be "vdd = sessionManager.DCPower"
• Fix typo in InstrumentAbstraction.md, should be “InitializeAndClose” class not “Initialization”
• Update DataAbstraction.md to highlight structure and debug experience
• Add note to WritingTestCode.md to denote prerequisites and that the steps are typical not always required
• Add new user guide topic for workflow of customizing TestStandSteps (and explaining new Shared.cs)

Why should this Pull Request be merged?

This PR provides quality improvements and addresses feedback received from users.

What testing has been done?

These are just documentation changes, no testing done or required outside of basic spelling and grammar checks.