"Security Doucmentation" --> "Security Documentation"

Some more details about creating a settings page for an extension.

Three changes here:
 * Sort criteria in the same order across all three templates.
 * For meta-instructions, use the Markdown/HTML comment notation.
 * For `r-run`, we really don't want boilerplate. Remove the boilerplate.

The DEL workflow produces more readable output.

This makes it easier to find for people who come to the page just
looking for this one thing, which should be a common use-case of this
content.

There was confusion in some docs about `r-user` vs `r-users`. Standardize on `r-user`.

The name `r-technical` was bit more verbose than all the other abbreviations. `r-tech` looks like it'll fit better.

style guide ref:
https://docs.civicrm.org/dev/en/latest/documentation/style-guide/#hackable-urls

Typo

Changed the line numer in the link of the example file for declaring the settings folder.

https://github.com/civicrm/civicrm-dev-docs/pull/425#issuecomment-338253390

Page should not link to itself.

I'd like to be able to merge the general, incremental improvements for the
"Testing" section, but these two new pages aren't yet written.  Comment them
out.