Button and toast guidelines

[gchaps]

Adds the following:

• Guidelines for buttons.
• New prop for GuideRuleExample of frame which will add some styling for siloed content.
• Adds ability to hide routes from the sidenav. I (Dave) was using something like this for a sandbox earlier, but figured its useful here for the toasts / modal docs were are ok to publish, but still need some cleaning before I'd want them public. Having them in master though means we can point to them and talk about them more easily as a team.

TODO

• @gchaps needs to copyedit the buttons guideline.
• Dave needs to clean up the other guidelines (in another PR).

[cchaos]

There are some really great points in here, I think a few could use a bit of a better description to understand the do/don't of it.

In terms of designing the page, we should figure out how we want to handle screenshots generally. I also wonder if it would help to add a TOC (or just sub sections on the left sidebar).

For the labels section, I would stick to always showing them in button form or not at all (I prefer the former option) and then maybe adding a second column for Good/bad with green/red icons to better discern between good/bad.

[snide]

OK. I cleaned this one up pretty heavily. For now I'm gonna hide toast and modal guidelines from the side nav till they can get the same treatment, but at this point I'd consider it mergeable minus some copyediting.

[snide]

Worked through this stuff with gail over a couple sessions. This is good to go.