revised layout

[mbostock]

Changes the header to be top-fixed and to extend all the way to the right edge, and moves the table of contents to under the header. Changes the layout on wide windows so that the sidebar and table of contents remain attached to the center column rather than being on the far edges of the window. Moves the header styles to the default theme from our docs config so that the header works “out of the box” without users having to copy these styles from the docs repo (if they chose to have a header).

Before:

After:

Fixes #657.
Supersedes #1023.

[mbostock]

Also… wondering if there is a quick fix we can do for the table of contents on mobile #1136. Maybe we can just show it inline rather than position-fixed on narrow windows, so that you can quickly jump down from the top to the desired section? Going to punt on this for now since it’s a separable concern.

[CobusT]

LGTM! Thanks.