Fill in @defgroup/topic docs for Best Practices overview

On #2148255: [meta] Make better D8 api.d.o landing page, linked to high-level overview topics, and put it in Core api.php files, we made a patch that included a stub Topic page for api.drupal.org (i.e., a @defgroup) titled:

Best Practices for Developers

This can be found in file core/modules/system/core.api.php where it says

@defgroup best_practices

The documentation to go on this page needs to be written. The idea is:

a) Write a few paragraphs about the topic.

b) Link to more detailed documentation on
https://drupal.org/developing/api/8
(or nearby sections)

c) If the more detailed documentation does not yet exist, create stub page(s), link to the stub pages, and add a note to this issue stating that the stub pages need to be filled out.

d) If the topic has related classes, interfaces, and functions -- appropriate for an overview -- add

@ingroup best_practices

to their documentation headers. That will make these classes etc. show up on the Topic page on api.drupal.org. Only include classes/functions that are appropriate for an overview page please!

For more info -- documentation standards for @defgroup/@ingroup:
https://drupal.org/coding-standards/docs#defgroup