Fill in @defgroup/topic docs for Object Oriented Programming overview

So my one beef with this is it puts the most critical information (you need to put your classes just so or nothing happens) at the end, and emphasizes the least critical (FYI, we have coding standards for this) at the top. Personally, I would more or less invert the list:

+ * - Each class must be in its own file.
+ * - In order for the PSR-4-based class auto-loader to find the class, it must
+ *   be located in a directory corresponding to the namespace. For
+ *   module-defined classes, if the namespace is \Drupal\module_name\foo\bar,
+ *   then the class goes under the main module directory in directory
+ *   src/foo/bar. For Drupal-wide classes, if the namespace is
+ *   \Drupal\Core\foo\bar, then it goes in directory
+ *   core/lib/Drupal/Core/foo/bar. See https://www.drupal.org/node/2156625 for
+ *   more information about PSR-4.
+ * - Classes must be namespaced. If a module defines a class, the namespace
+ *   must start with \Drupal\module_name. If it is defined by Drupal Core for
+ *   use across many modules, the namespace should be \Drupal\Core or
+ *   \Drupal\Component, with the exception of the global class \Drupal. See
+ *   https://www.drupal.org/node/1353118 for more about namespaces.
+ * - Plugin discovery requires particular namespaces for most plugin classes.
+ *   See the @link plugin_api Plugin API topic @endlink for more information.
+ * - Some classes have annotations added to their documentation headers. See
+ *   the @link annotation Annotation topic @endlink for more information.
+ * - There are project-wide coding standards for OO code, including naming:
+ *   https://drupal.org/node/608152
+ * - Documentation standards for classes are covered on:
+ *   https://www.drupal.org/coding-standards/docs#classes

Something like that.

Additionally, while I definitely don't want to duplicate documentation found elsewhere here, it'd be nice if we could point to a reference (off-site or otherwise) that explains what the heck these newfangledy words (class, interface and especially trait) are. Not just for D6/7 > D8 migrators, but for PHP devs in general who've come from a procedural background.

Awesome, that looks great to me.

Since this was just a simple re-ordering of what was already there + the addition of a new link, I feel comfortable going ahead and committing this since it's already been at needs review for about a week.

Committed and pushed to 8.x. Thanks!