docs: Update module items in site nav

Moves module_structure.rst next to the module docs. A few weeks ago I
moved this into the contributing section and got feedback that it's not
a good place for that doc.

Renames "Module Guides" to "Modules". After SEED-0102, "guides" now has
a more narrow meaning. Labeling the section "Module Guides" might
mislead people into thinking that the section only contains how-to
guides.

Change-Id: I3940d4987ed262b622505fe00123463178b71e36
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/163251
Reviewed-by: Kayce Basques <kayce@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Kayce Basques <kayce@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/docs/contributing.rst b/docs/contributing.rst
index 47b3b97..dea4391 100644
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -432,4 +432,3 @@
 
   embedded_cpp_guide
   style_guide
-  module_structure
diff --git a/docs/index.rst b/docs/index.rst
index 3735c95..18adcf3 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -11,7 +11,8 @@
   docs/getting_started
   docs/concepts/index
   targets
-  module_guides
+  Modules <module_guides>
+  docs/module_structure
   changelog
   Mailing List <https://groups.google.com/forum/#!forum/pigweed>
   Chat Room <https://discord.gg/M9NSeTA>