Add a class to show what nav link is selected

Created by: sdwilsh

This change only adds the selection for the Getting Started section for now. More to come in later changes (trying to keep diff size reasonable)

This change also adds some CSS to differentiate the selected navigation target.

Test Plan: Ensured that following pages were properly selected when viewed:

• http://localhost:9811/about/overview.html
• http://localhost:9811/setup/install.html
• http://localhost:9811/setup/intellij_plugin_install.html
• http://localhost:9811/setup/getting_started.html
• http://localhost:9811/article/exopackage.html
• http://localhost:9811/article/query_cheat_sheet.html