Remove second-level headings from the dev doc + hide TOC

The only two titles at the second level were 'Important documentation links'
and 'Content', which look weird and are not very useful.

And this level prevented headings inside package documentation from being
shown in the TOC at all, which means the TOC couldn't be used to navigate
within a package's documentation.

docs/devel/index.rst

@@ -3,11 +3,23 @@
 Software Heritage - Development Documentation
 =============================================
 
-Important documentation links
------------------------------
+.. toctree::
+   :hidden:
+
+   getting-started/index
+   architecture/index
+   contributing/index
+   tutorials/index
+   faq/index
+   roadmap/roadmap-2022
+   roadmap/index
+   api-reference
+   archive-changelog
+   journal
+   statsd
 
 Getting started
-^^^^^^^^^^^^^^^
+---------------
 
 * :ref:`getting-started` → deploy a local copy of the Software Heritage
   software stack in less than 5 minutes, or
@@ -16,7 +28,7 @@ Getting started
 * :ref:`faq`
 
 Contributing
-^^^^^^^^^^^^
+------------
 
 * :ref:`patch-submission` → learn how to submit your patches to the
   Software Heritage codebase
@@ -25,7 +37,7 @@ Contributing
 * :ref:`python-style-guide` → how to format the Python code you write
 
 Architecture
-^^^^^^^^^^^^
+------------
 
 * :ref:`architecture-overview` → get a glimpse of the Software Heritage software
   architecture
@@ -33,14 +45,14 @@ Architecture
   stores and handles metadata
 
 Data Model and Specifications
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+-----------------------------
 
 * :ref:`persistent-identifiers` Specifications of the SoftWare Heritage persistent IDentifiers (SWHID).
 * :ref:`data-model` Documentation of the main |swh| archive data model.
 * :ref:`journal-specs` Documentation of the Kafka journal of the |swh| archive.
 
 Tutorials
-^^^^^^^^^
+---------
 
 * :ref:`testing-guide`
 * :doc:`tutorials/issue-debugging-monitoring`
@@ -50,7 +62,7 @@ Tutorials
 * :ref:`doc-contribution`
 
 Roadmap
-^^^^^^^
+-------
 
 * Current roadmap: :ref:`roadmap-current`
 * Previous roadmaps
@@ -58,7 +70,7 @@ Roadmap
   * :ref:`roadmap-2021`
 
 System Administration
-^^^^^^^^^^^^^^^^^^^^^
+---------------------
 
 * :ref:`Network Infrastructure <infrastructure>`
 * :ref:`mirror` → learn what a Software Heritage mirror is and how to set up
@@ -69,7 +81,7 @@ System Administration
 .. _components:
 
 Components
-^^^^^^^^^^
+----------
 
 Here is brief overview of the most relevant software components in the Software
 Heritage stack, in alphabetical order.
@@ -193,7 +205,7 @@ swh.docs
 
 
 Dependencies
-^^^^^^^^^^^^
+------------
 
 The dependency relationships among the various modules are depicted below.
 
@@ -206,29 +218,11 @@ The dependency relationships among the various modules are depicted below.
 
 
 Archive
-^^^^^^^
+-------
 
 * :ref:`Archive ChangeLog <archive-changelog>`: notable changes to the archive
   over time
 
-Contents
---------
-
-.. toctree::
-   :maxdepth: 2
-
-   getting-started/index
-   architecture/index
-   contributing/index
-   tutorials/index
-   faq/index
-   roadmap/roadmap-2022
-   roadmap/index
-   api-reference
-   archive-changelog
-   journal
-   statsd
-
 
 .. only:: devel_doc