Clarify and divide README

[nikimanoledaki]

What this PR does / why we need it:

This PR:

• Clarifies and divides the README into logical sections to help a new user understand what the project, specification, library, and CLI are about.
• Adds a link to the CNCF Code of Conduct in a .md file.
• Moves documentation links to ocm-spec so that we can make that the SoT (not ocm/docs).

Which issue(s) this PR fixes:
Fixes part of #98

Special notes for your reviewer:

After the conversation on docs, I split the work that I was doing with editing the docs to do it in 2 PRs. The other changes are being moved to the ocm-spec repository rather than the ocm repository.

Note: This is just one iteration of adding to the README and we will iterate on it further.

Release note:

[gardener-robot]

@nikimanoledaki Thank you for your contribution.