add documentation topic on entity hooks

Problem/Motivation

The entity system fires many hooks when an entity is created or updated:

- hook_entity_insert/update/pre_save
- hook_ENTITYTYPE_insert/update/pre_save

The documentation for some of these is unclear about the precise order in which they are invoked, the status of the entity at that point (ie, has it been saved yet? does it have an ID yet?), and what it's legitimate to do at that point (may the entity be altered?)

Proposed resolution

Add a new documentation topic for entity hooks. This should:

- give the order in which hooks are fired for basic entity operations such as insert, update, and delete
- mention there are both the common entity hooks and the specific entity type hooks

The documentation for all these hooks should be added to this topic.

Original report by joachim

API page: https://api.drupal.org/api/drupal/modules!system!system.api.php/function...

hook_entity_insert() is invoked just after an entity has been saved for the first time. This means it will have an ID.

The docs could also do to point the reader at hook_entity_presave(), in case that is better suited to what they're looking for.