Dev Docs: Add "Edit|History|Report Issue|Discuss" Links To Subheads

Previews:
* http://dg2.dtrt.org/en/developer-guide
* http://dg2.dtrt.org/en/developer-reference
* http://dg2.dtrt.org/en/developer-examples

This pull adds contextual links below most subheadings.  They look like this:

![2014-12-13-175655_302x109_scrot](https://cloud.githubusercontent.com/assets/61096/5425730/7adaa564-82f1-11e4-8c41-3088f54343a7.png)


This helps solve a few minor problems with the current Javascript-based left-side links:

1. Javascript is not required for these new links to work fully. This is a really minor issue since JS is still required to get rid of the BETA warning pop-up.

2. These new links are much more visible than the old links on long sections. This was discussed on issue #647

3. When sections from two different files are both displayed at the same time, clicking the old Edit or Issue links may operate on the wrong file.

Number 3 was my main motivation.  On a WIP branch, I've split each of the RPC subsections into a different file, greatly increasing the chance a prospective editor/bug reporter will get the wrong context.

In addition, I think having the links displayed under every section will gently remind users over and over that they too can contribute, or point them in the direction of interactive help if they're confused.

I'm sure this pull can be improved upon, so please let me know what ya'll think.