Represent nested deps in targets.yaml along with title styles

This will allow us to programatically position .rst snippets *anywhere*
which will for once and for all remove the horrid title level mismatch bugs.

We require this in order to allow people to re-shuffle the spec without
having to adjust the spec itself (e.g. 2 targets with different levels of
nesting).

specification/targets.yaml

@@ -20,6 +20,10 @@ groups:
     - modules/04_00_content_repo.rst
     - modules/05_00_end_to_end_encryption.rst
     - modules/06_00_history_visibility.rst
-    - modules/07_00_push_overview.rst
-    - modules/07_01_push_cs_api.rst
-    - modules/07_02_push_push_gw_api.rst
+    - 1: modules/07_00_push_overview.rst
+      2: [modules/07_01_push_cs_api.rst , modules/07_02_push_push_gw_api.rst]
+title_styles:
+ - "="
+ - "-"
+ - "~"
+ - "+"