Improve getting started doc section

[Wumpf]

What

• Fixes Simplify Getting Started Section of Documentation #5460

Various improvements to the getting started doc sections:

• better layout / new nesting structure
• improved titles, updated some references accordingly
• better & more complete install instructions
• remove 'Welcome' page, make index == new "What is Rerun?" page

⚠️ adds a placeholder page for getting started on blueprint

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[emilk]

Great

[github-actions]

Size changes

Name	main	5689/merge	Change
Wasm	18.67 MiB	20.03 MiB	+7.28%