Make section headings more navigable

[victorlin]

This was brought up in nextstrain/augur#1243 (comment) and also on Slack some time ago.

Currently, it's up to individual pages to provide a table of contents for navigation between sections of long pages. Example (rendered):

docs.nextstrain.org/src/install.rst

Lines 18 to 19 in c0e1eb9

	.. contents:: Table of Contents
	:local:

A better solution would be something like a sidebar with interactive headings (example). This has the following benefits over the existing RTD-generated table of contents:

1. It is always visible, even when clicking on a section heading and jumping to that part of the document.
2. It highlights the currently visible part of the document.

These 2 features allow readers to know where they are in relation to the entire document and provides a better experience for documents with many headings/subheadings.