Add cross-references

This completes #47 by adding cross-references throughout the document.  The changes made include:

* Adding over 950 internal links, creating a dense web of cross-references which allow readers to quickly jump to relevant information

* Adding a `title` (tooltip) to each internal link so readers can quickly get definitions to unfamiliar (or forgotten) terms.

* Adding a few lines to one of the stylesheets to make the internal links more subtle. This is just a quickly implemented idea; I expect @saivann will find the right style.

* Some miscellaneous edits. Because I had to re-read the entire document twice to create all these cross-references, I make some small changes.  I only made changes I thought would be non-controversial, such as fixing grammar mistakes and minor technical errors.

* Contrary to my proposal, I added the links to the bottom instead of the top. I think this makes contributing to the document for the first time less intimidating, and will hopefully help when we make the pull request to the master bitcoin.org repository.

Leaving this giant update unmerged will create conflicts---which I'm happy to resolve---but, if @saivann agrees, I suggest we merge it as soon as he runs a test Jekyll rebuild. We can address any problems afterwards (and roll back the change if necessary).