[prototype/WIP] widened side grid

[j-low]

[description]:

• rough draft for what a widened site grid would appear as on docs

[cockroach-teamcity]

This change is 

[j-low]

cc: @jseldess

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/1f5d2e0b892fc1be15f21f702fb2d6163af10b8d/

[jseldess]

This is cool, @j-low. One quick comment: Instead of letting the main body expand to the right indefinitely, I think we want a max width. While we're doing this, we should move the page-level TOC to a right margin. @kuanluo mocked this up a bit here: #2195 (comment)

Some decent examples of using the right margin for page TOC:

• https://developers.google.com/identity/protocols/OAuth2
• https://valelint.github.io/docs/styles/#creating-a-style

[j-low]

@jseldess sure thing...there will be progressive updates to this PR. I'll be working on that third column for the TOC next.

[jseldess]

For clarity, I'm closing this in favor of the more recent #3284.