Restructure ToC

What has changed

The ToC shown on the index page contained links to sections which should not belong on the top level.

I have restructured the ToC so that from the top level we see:

• Installing BuildStream
  • Note: This now includes "BuildStream inside Docker"
• Using BuildStream
  • Note: This now includes "Authoring Projects"
• General Documentation
  • Note: This is a new section which contains "Artifact caches", "Cache Keys" and "Sandboxing"
• Core documentation and references
  • Note: This now contains: "buildstream" under "internals"
• Resources
• Hacking on BuildStream

The ToC tree captions have also been removed as they served no purpose.

Result

What I would like to discuss

1. Thoughts on the "General Documentation" section? Or should we incorporate Artifact Caches, Cache Keys and Sandboxing into the "Core documentation and reference" section?
2. Thoughts on including "Authoring Projects" under the Section "Using BuildStream"?
3. We no longer have section headings on the homepage, thoughts?