[DOCS][Developer] Remove low level index, we will approach these themes in another manner

Fix some broken links

docs/developer/src/SUMMARY.md

@@ -11,7 +11,7 @@
 - [Internationalization](./i18n.md)
 - [Logging](./log.md)
 - [Queue](./queue.md)
-- [Attachments, Files and Thumbnails](./attachments.md)
+- [Attachments, Files and Thumbnails](./storage.md)
 - [Security](./security.md)
 - [HTTP Client](./httpclient.md)
 - [Plugins](./plugins.md)
@@ -22,16 +22,4 @@
     - [Injecting Javascript](./plugins/sample_plugins/Awesomeness.md)
     - [Creating a block on the sidebar](./plugins/sample_plugins/Sample.md)
 - [Low level](./core.md)
-    - [Overview](./core/overview.md)
+    - [Overview](./core/overview.md)
-    - [Modules](./core/modules.md)
-    - [Event dispatcher](./core/events.md)
-    - [ORM and Caching](./core/orm_and_caching.md)
-    - [Interfaces](./core/interfaces.md)
-    - [UI](./core/ui.md)
-    - [Internationalisation](./core/i18n.md)
-    - [Utils](./core/util.md)
-    - [Queues](./core/queues.md)
-    - [Files](./core/files.md)
-    - [Security](./core/security.md)
-    - [HTTP Client](./core/http.md)
-    - [Exception handling](./core/exception_handler.md)

docs/developer/src/architecture.md

@@ -14,10 +14,10 @@ which is elaborated in [Database](./database.md);
 - [Internationalization (i18n)](https://en.wikipedia.org/wiki/Internationalization_and_localization), elaborated in [Internationalization](internationalization.md);
 - [Exceptions](./exceptions.md);
 - [Log](./log.md);
-- [Queues](./queues.md);
+- [Queues](./queue.md);
-- [Files](./files.md);
+- [Storage](./storage.md);
-- [Sessions and Security](./sessions_and_security.md);
+- [Sessions and Security](./security.md);
-- [HTTP Client](./http.md).
+- [HTTP Client](./httpclient.md).
 
 Everything else uses most of this.
 

docs/developer/src/core.md

@@ -1,19 +1,6 @@
 # The Core
 This documentation adopted a top-down approach. We believed this to be the most helpful as it reduces the time needed
 to start developing third party plugins. To contribute to GNU social's core, on the other hand, it's important to
-[understand its flows](./core.md) and internals well.
+[understand its flows](./core/overview.md) and internals well.
 
 The `core` tries to be minimal. The essence of it being various wrappers around Symfony. It is divided in:
-
-- [Modules](./core/modules.md);
-- [Event dispatcher](core/events.md);
-- [ORM and Caching](./core/orm_and_caching.md);
-- [Interfaces](./core/interfaces.md);
-- [UI](./core/ui.md);
-- [Internationalization](core/i18n.md);
-- [Utils](./core/util.md);
-- [Queues](./core/queues.md);
-- [Files](./core/files.md);
-- [Sessions and Security](./core/security.md);
-- [HTTP Client](./core/http.md).
-- [Exceptions](./core/exception_handler.md).