docs: Various documentation improvements

Which issue does this PR close?

N/A

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