v4: Grid adjustments

Grouping a handful of CSS and docs changes related to the grid:

• Moves from max-width containers to width, with a max-width: 100%. This corrects inconsistencies between browsers and default/flexbox grid implementations; previously some containers wouldn't occupy their full max-width. Fixes #20681 (closed).
• Changes the values of the sm breakpoint and container. Now the breakpoint is wider than the container (this was a longstanding bug from me, sorry about that 😁). Fixes #18054 (closed).
• Changes the large grid breakpoint's value from 940px to 960px so that every grid tier, in it's max width, is cleanly divisible by 12. Fixes #18510 (closed).
• Update grid docs to match all the above changes.