Add next and previous buttons

[heatherbooker]

Hi! I just started reading the textbook, and I think it would be great to have a way to go to the "next" section (based on the table of contents), from any given page.

Maybe that could be:

• a link at the end of the content, beside the "share" button
• "next" and "previous" links at the bottom
• links at the bottom containing the title of pages

Both the second methods are used for example in postgresql docs if you scroll to the bottom.

In keeping with the simplicity of the textbook currently, I like the first option. Here's how it could look:

What do you think? :)

[bkushigian]

I second this (and I was just talking to a friend who thirds it!!)