Improve documentation on doc.opentap.io

• General improvements:

• Getting Started: Don’t include sub chapters for “Use OpenTAP” and “Developer Plugins”

• Cli reference: Fix the table, the hierarchy is not visible

• Improvements in Developer Guide:

• Add a Getting Started Page with high level overview of development workflow and tools.

• Rename “Getting started with your own project” to “Using the OpenTAP Nuget package”

• Rename “Getting Started with the Example Project” to “OpenTAP SDK Examples”

• Add “OpenTAP SDK Templates” section