Make the toctree match the 'Getting started' and 'Architecture' sections of the main page

We have two hierarchies and they didn't match before this commit.

This also keeps the root toctree shorter (2 items instead of 4)

docs/architecture/index.rst

+Architecture
+============
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+   :titlesonly:
+   :hidden:
+
+   architecture
+   mirror

docs/getting-started-index.rst

+Getting started
+===============
+
+.. this index file is used to force a hierarchy in the toctree,
+   even though these two files are in the same directory
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+   :titlesonly:
+   :hidden:
+
+   getting-started
+   developer-setup

docs/index.rst

@@ -171,9 +171,8 @@ Indices and tables
    :titlesonly:
    :hidden:
 
-   architecture
-   getting-started
-   developer-setup
+   getting-started-index
+   architecture/index
    contributing/index
    tutorials/index
    API documentation <apidoc/modules>