engineering: Core Application team updates

Created by: tsenart

This commit introduces a number of changes:

1. De-list frontend role from open positions. A new manager, once hired, can then focus on hiring additional teammates. @withdavidli: What steps do we need to take in Lever?
2. Rename Cloud to Core application everywhere.
3. Update and simplify processes section in the team page.
4. Outline the two different work streams (core application and backend platform).
5. Update growth plan.