Cleanup README file

[jeanduplessis]

Description of your changes

Various cleanup fixes to the README file:

• Add command to install latest build version from main branch
• Remove outdated information and broken links
• Remove usage commands in favor of pointing to docs.upbound.io

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] ~Added backport release-x.y labels to auto-backport this PR, as appropriate.~

How has this code been tested

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