doc update

[ksator]

update the list (some are missing: ygot, ansible ...) https://github.com/aristanetworks/openmgmt/blob/main/docs/examples/index.md https://aristanetworks.github.io/openmgmt/examples/

update the README file (moving contributions details in an external contribution guide)

[sulrich]

per discussion on 20220121

• move the contributing guidelines from the README and move these into a separate document
• the README should really look like a point of entry that outlines what people can learn from this repo.

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