Docs: Add ADRs subsection to "For Developers"

docs/arch/index.md → docs/developers/arch/index.rst

-Architecture decision records
+Architecture Decision Records
 =============================
 
 Here you will find documents which describe significant architectural decisions
@@ -6,6 +6,11 @@ made or proposed when developing the Hypothesis client. We record these in
 order to provide a reference for the history, motivation, and rationale for past
 decisions.
 
-See [the
-introduction in the "h" repository](https://github.com/hypothesis/h/blob/master/docs/arch/index.rst)
+See
+`the introduction in the h repository <https://github.com/hypothesis/h/blob/master/docs/arch/index.rst>`_
 for more details.
+
+.. toctree::
+   :maxdepth: 1
+
+   adr-001

docs/developers/index.rst

@@ -5,8 +5,9 @@ This section contains documentation for **developers** contributing code to
 the Hypothesis client.
 
 .. toctree::
-   :maxdepth: 3
+   :maxdepth: 2
 
    developing
    security
    envvars
+   arch/index