updates and consolidation of hiring guidelines

[smola]

We have two guide for interviews:

• The application process guide, meant to be used by candidates.
• The hiring process guide, which document the process for interviewers at source{d} (at least those parts that can be publicly disclosed).

Some sections were duplicated and eventually got out of sync. Changed the internal guide to link to the application process guide whenever the information was redundant. In some cases, merged both sections.

Additionally:

• Added C++ option to the coding challenge.
• Added the remote option for the final round of interviews.