docs: Various documentation improvements

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Various documentation improvements:

• Installation Guide:
  • Remove old content stating that we do not have published releases yet
  • Add direct links for downloading jar files from Maven
  • Move instructions for building from source to a separate guide to keep the main installation guide more concise
  • Enable shuffle in instructions for using Comet
  • Enable off-heap memory in instructions for using Comet
• Comet plugin overview in user guide:
  • Use consistent language from README when describing the product
• Comet plugin overview in contributor guide:
  • Use new versions of diagrams taken from recent CMU talk

What changes are included in this PR?

How are these changes tested?