Add hook_examples module to demonstrate implementing, defining, and invoking hooks

The one thing I might recommend adding would be an event-based example (i.e., implement an event and a listener), ideally for the same example you do the hook for. We're moving slowly away from hooks now that we can do listeners easily, and having the example show how to do the same job using both mechanisms is a good way to make people aware this exists as an option.

First of all, for a first draft, this is really good. Well worded, and if you didn't already run this through phpcs, you're the most careful coder I've ever seen :-)

I have a few minor quibbles, some of which you should feel free to ignore. But in general: this code uses our best practices, and has a meaningful *phpunit* based test. Not much that you need to improve here.

1. +++ b/hooks_example/hooks_example.module
   @@ -0,0 +1,226 @@
   + * Hooks are specially named functions called at critical points in order to
   

   Not sure if I can come up with a better word than "critical", but if you can think of one, worth it.

2. +++ b/hooks_example/hooks_example.module
   @@ -0,0 +1,226 @@
   + * Every hook has three parts; A name, an implementation, and a definition.
   

   "a name"

3. +++ b/hooks_example/hooks_example.module
   @@ -0,0 +1,226 @@
   + * When implementing a hook you can use the standard "Implements HOOK_NAME."
   

   "should use" -- it's a standard.

4. +++ b/hooks_example/hooks_example.module
   @@ -0,0 +1,226 @@
   +  $session = \Drupal::request()->getSession();
   

   Alternatively, you could use the State API here. Not a problem, but would demonstrate a good practice.

On second thought: yeah, use the session. Separating users in state would be a mess.

Much thanks, and a happy new year.