Add documentation landing and quick start

[lipkau]

Summary

• replace the placeholder docs landing page with useful entry points
• add active module status/reference matrix
• add a Quick Start page for install, import, authentication, and command discovery
• add Quick Start to documentation navigation

Validation

• bundle exec jekyll build --baseurl ""

Stacked on #51.

[lipkau]

Closing this stacked PR so PR #51 can be recycled for the consolidated website redesign.