README improvements; fixes https://github.com/cockroachdb/docs/issues/1189
README improvements; fixes https://github.com/cockroachdb/docs/issues/1189

- remove explicit commands from Quickstart; link to relevant docs instead
- link to Build an App tutorials in Client Drivers section
- change Cloud Deployment link to point to docs instead of to github
- link to Orchestration docs in Deployment section

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