DOC: Add autodoc section "Ensuring the code can be imported" (#12426)

This explains the requirements for importablilty (can be found and dependencies can be resolved). It also explains two common approaches how this can be achieved.

doc/usage/extensions/autodoc.rst

@@ -12,13 +12,6 @@
 This extension can import the modules you are documenting, and pull in
 documentation from docstrings in a semi-automatic way.
 
-.. note::
-
-   For Sphinx (actually, the Python interpreter that executes Sphinx) to find
-   your module, it must be importable.  That means that the module or the
-   package must be in one of the directories on :data:`sys.path` -- adapt your
-   :data:`sys.path` in the configuration file accordingly.
-
 .. warning::
 
    :mod:`~sphinx.ext.autodoc` **imports** the modules to be documented.  If any
@@ -44,6 +37,39 @@ docstrings to correct reStructuredText before :mod:`autodoc` processes them.
 .. _NumPy: https://numpydoc.readthedocs.io/en/latest/format.html#docstring-standard
 
 
+Ensuring the code can be imported
+---------------------------------
+
+:mod:`~sphinx.ext.autodoc` analyses the code and docstrings by introspection after
+importing the modules. For importing to work. you have to make sure that your
+modules can be found by sphinx and that dependencies can be resolved (if your
+module does ``import foo``, but ``foo`` is not available in the python environment
+that Sphinx runs in, your module import will fail).
+
+There are two ways to ensure this:
+
+1. Use an environment that contains your package and Sphinx. This can e.g. be your
+   local dev environment (with an editable install), or an environment in CI in
+   which you install Sphinx and your package. The regular installation process
+   ensures that your package can be found by sphinx and that all dependencies are
+   available.
+
+2. It is alternatively possible to patch the Sphinx run so that it can operate
+   directly on the sources; e.g. if you want to be able to do a sphinx build from a
+   source checkout.
+
+   - Patch :data:`sys.path` in your Sphinx :file:`conf.py` to include the folder of
+     your sources. E.g. if you have a repository structure with :file:`doc/conf.py`
+     and your package is at :file:`src/mypackage`, then you sould add::
+
+        sys.path.insert(0, os.path.abspath('../src'))
+
+     to your :file:`conf.py`.
+
+   - To cope with missing dependencies, specify the missing modules in
+     :confval:`autodoc_mock_imports`.
+
+
 Directives
 ----------