feature #8729 Doctrine update for Flex (weaverryan)

This PR was squashed before being merged into the 4.0 branch (closes #8729).

Discussion
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Doctrine update for Flex

Hi guys!

This is an update to 4.0. But... I basically rewrote the main `doctrine.rst` page and also `doctrine/associations.rst`. All the rest is just a simple directory structure update. I made many changes, but I DID code through all of them, so we don't need to review the code for "will this actually work?" type of stuff :).

* [ ] verify that the section where we talk about "line 12 does this" is still accurate to the lines

Commits
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39c4a83 fixes thanks to the team!
04603c7 Fixing build problems
59c6b0d tweaks thanks to @yceruto
2b0236f updating the rest of the Doctrine docs
68ac454 WIP - updating associations article
7e77d9c moving section down
6259931 Finishing major overhaul of the main Doctrine chapter
4938678 WIP - starting to upgrade Doctrine doc

Author: weaverryan

_build/redirection_map

@@ -138,18 +138,21 @@
 /cookbook/deployment/platformsh /deployment/platformsh
 /cookbook/deployment/tools /deployment/tools
 /cookbook/doctrine/common_extensions /doctrine/common_extensions
-/cookbook/doctrine/console /doctrine/console
+/cookbook/doctrine/console /doctrine
 /cookbook/doctrine/custom_dql_functions /doctrine/custom_dql_functions
 /cookbook/doctrine/dbal /doctrine/dbal
 /cookbook/doctrine/event_listeners_subscribers /doctrine/event_listeners_subscribers
 /cookbook/doctrine/index /doctrine
-/cookbook/doctrine/mapping_model_classes /doctrine/mapping_model_classes
+/cookbook/doctrine/mapping_model_classes /doctrine
+/doctrine/mapping_model_classes /doctrine
 /cookbook/doctrine/mongodb_session_storage /doctrine/mongodb_session_storage
 /cookbook/doctrine/multiple_entity_managers /doctrine/multiple_entity_managers
 /cookbook/doctrine/pdo_session_storage /doctrine/pdo_session_storage
 /cookbook/doctrine/registration_form /doctrine/registration_form
 /cookbook/doctrine/resolve_target_entity /doctrine/resolve_target_entity
 /cookbook/doctrine/reverse_engineering /doctrine/reverse_engineering
+/doctrine/repository /doctrine
+/doctrine/console /doctrine
 /cookbook/email/cloud /email/cloud
 /cookbook/email/dev_environment /email/dev_environment
 /cookbook/email/email /email