Document and update eng team structure and hiring plan

[nicksnyder]

This PR bundles a few changes:

1. Document who is on what team. This serves as our org chart. We can discuss re-orgs via PR in the future 🙃
2. Document our hiring needs for each team (instead of various Google Docs here and there).
3. Update our team structure to reflect that we are orienting work around Sourcegraph Cloud. You should read this as "core services now owns a superset of what they owned before" and the hiring plan is designed to reflect that.
4. Remove philosophical documentation around how our team is organized because (1) we decided to have engineering managers manage cross-functional engineers on a single mission, and (2) I think it is more effective to just document where we are today (and then evolve that over time).

TODO

• Update Slack channels, GitHub groups, GitHub labels, CODEOWNERS, etc. for Core services -> cloud change.

[tsenart]

@nicksnyder: Anything left to do here?

[nicksnyder]

@tsenart I renamed things in Slack, GitHub and CODEOWNERS to the best of my ability. If you see anything I missed, please take care of it :)