Update documentation for BD

[haroldcampbell]

• Added brief note about how identify which Engagements need renewing
• Restructured BD document hierarchy so that it's easier to find information

[choldgraf]

I'm going to merge this one because it seems reasonable and more like improving/rearranging rather than adding new policy. I suggest that y'all continue to improve these docs iteratively and just self-merge this kind of thing in the future.

[choldgraf]

Actually I take that back - some decisions in this PR don't make sense to me and they've broken the docs build.

@haroldcampbell why did you move a bunch of pages under a business-development/bd/ folder? What's the benefit of bd/?

If we do this, we'll need to update all relative paths in all of those pages so that they aren't broken. That's causing the docs build to fail.