Add investment time documentation

[hancush]

Overview

This PR adds the proposal, pilot, recommendation, and documentation of investment time at DataMade.

Handles https://github.com/datamade/hr/issues/26 (internal link)

Testing Instructions

• View the rendered markdown and confirm it looks good.
• Click the links and confirm they resolve correctly.

[hancush]

@derekeder I added some language about working with your manager on next steps if urgent issues come up during investment day. I'll also add that, that level of urgency is pretty rare, and in both cases, staffing is such that it could be handled by a partner during investment day, or on Friday by a staff member.

Also clarified that investment time is a function of hours worked. This policy makes it easy to accommodate different schedules: You simply calculate your allowance from time worked in the previous week, plus time scheduled in the week of investment day, and use that many hours on investment day.

[hancush]

This is ready for re-review, @derekeder, @fgregg, and @jeancochrane. Thank you so much, all, for your thoughtful discussion on scheduling.

[hancush]

Thanks, team!