Document differences between burndown and burnup charts

Follow-up from discussion in !44684 (comment 426681930):

The in common section is more of a high-level description of the charts. I think separate 'how it works' sections for each where we delve into some specifics. There'll be a little duplication between the two sections but I think it's better than "read what they have in common, then read things specific to this type of chart".

Implementation guide

We could maybe move things like the total/weight toggle to shared section.

In common:

• both charts show milestone progress
• both show total number of issues assigned to the current milestone for each day of the milestone
• both can show either total issues, or total weight

Differences:

• burnup charts have a separate line showing when issues are completed (closed)
• burnup charts show the difference between an issue being moved to another milestone ('total issues' goes down) and an issue being closed ('total issues' stays the same, 'completed issues' goes up)
• burndown charts show "total issues - completed issues" each day, burnup charts "total issues" line is the total (open and closed issues)