docs: Add missing index.rst for documentation/.

Apparently I failed to commit this part of our documentation
migration.

docs/documentation/index.rst

@@ -0,0 +1,12 @@
+#####################
+Writing Documentation
+#####################
+
+.. toctree::
+   :maxdepth: 3
+
+   overview
+   user
+   integrations
+   openapi
+

docs/index.rst

@@ -39,7 +39,7 @@ Contents:
 * :ref:`Code Contribution Guide <code-contribution-guide>`
 * :ref:`Code Testing <code-testing>`
 * :ref:`Subsystem Documentation <subsystem-documentation>`
-* :ref:`Writing Documentation <documentation>`
+* :ref:`Writing Documentation <writing-documentation>`
 * :ref:`Translating <translating>`
 
 .. _overview:
@@ -98,7 +98,7 @@ Contents:
 
    subsystems/index
 
-.. _documentation:
+.. _writing-documentation:
 
 .. toctree::
    :maxdepth: 3

docs/subsystems/index.rst

@@ -35,14 +35,10 @@ Subsystems Documentation
    django-upgrades
    release-checklist
    api-release-checklist
-   openapi-api-docs
    conversion
    input-pills
    thumbnailing
    presence
    unread_messages
    billing
-   documentation
-   user-docs
-   integration-docs
    widgets