core-services: refresh team page

[bobheadxi]

Our team page is quite naked at the moment, this gives it a much-needed refresh:

1. Team abstract + mid-term goals, based loosely on @rafax's proposed charter a while ago and some stuff off the top of my head
2. Ownership areas, which links to go/ship-okrs but also gives a high-level overview of things we own
3. Processes, which covers: Team planning, Issue tracking and Project views, and Recurring meetings` to start
4. Refresh the team managed services page, renaming it to Core Services Managed Services to avoid confusion with the other ones

Preview: https://deploy-preview-8795--sourcegraph-handbook.netlify.app//departments/engineering/teams/core-services/

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[sourcegraph-bot]

Notifying subscribers in CODENOTIFY files for diff 855db4a...cea9a02.

Notify	File(s)
@unknwon	content/departments/engineering/teams/core-services/index.md content/departments/engineering/teams/core-services/managed-services/index.md content/departments/engineering/teams/core-services/managed-services/platform.md

[unknwon]

Much deserved update, thank you!