add docs for contributing and enhancement proposals

[reginapizza]

What type of PR is this? /kind documentation

What does this PR do / why we need it: In the spirit of making this project more open-source and friendly to new contributors, we need better documentation on contributing and more open process for submitting enhancement proposals. The decision to move enhancement proposals here was a result of a discussion during the team GitOps cabal on 09/05/22.

Have you updated the necessary documentation?

• [x] Documentation has been updated.

How to test changes / Special notes to the reviewer:

• check out to PR branch
• follow directions in https://argocd-operator.readthedocs.io/en/latest/contribute/development/#build-and-verify-argo-cd-operator-docs