This is a follow-up to [!347] to fix the unresolved review issues and
any other small nits along the way. Specifically:

- README: Replace intro prose with features bullet-list.
- README: Reorder Java downloads to list Adoptium first and Spack last.
- README: Remove insufficient "generic" install instructions.
- INSTALL: Add instructions on how to run HPCViewer after installing it.
- INSTALL: Improve command sequences to install on Linux.
- INSTALL: Add instructions to install via Spack.
- CONTRIBUTING: Correct a few copy-pastes from the HPCToolkit repo.

[!347]: !347



Signed-off-by: Jonathon Anderson <anderson.jonathonm@gmail.com>