Reorganize primary navigation

[droumis]

extracted from #351

Reorganize primary navigation

• Remove 'Introduction'; users can get to the homepage by clicking on the holoviz logo
• Merge 'Installation' content into the Tutorial 'setup' Update install instructions #400
• Move 'Talks, 'Background', 'Tutorial', 'FAQ, into a new 'Learn' page. The current Talk page should become just one of several listed talks.
• Move 'Roadmap' as a subpage of 'Contribute'
• Rename 'Topics' as 'Topic Gallery' as 'Examples'
• Create a new 'Start Here' page for new content on choosing the appropriate HoloViz tools
• Create a new 'Blog' link
• Contingent on being able to customize the 'More' word dropdown... Replace the 'More' Dropdown with something like 'About', and nest new pages such as 'Governance', 'HEPs', 'Roadmap'. If we cannot customize 'More', nest these new pages into 'About'. Either way, ensure things work as expected on mobile.
• Order the revised main nav as follows: Start Here, Learn, Gallery, Community, Contribute, Blog, About

Nav proposal:

• Start Here (WIP)
• Learn
  • Background
  • Talks
  • Tutorial
• Examples (link to examples site)
• Community
• Contribute
• Blog (link to blog site)
• About
  • Governance
  • HEPs
  • Roadmap

[maximlt]

Rename 'Topics' as 'Topic Gallery' as 'Examples' Gallery

I think I'd prefer Examples since it's the domain name of examples.holoviz.org.