Add external snap docs and clean up dev docs (#8356)

* Add external snap docs and clean up dev docs

* Correctly refer to content identifiers

* Expand plugin interface docs and add line breaks

certbot/docs/contributing.rst

@@ -303,7 +303,7 @@ configuration checkpoints and rollback.
 .. _dev-plugin:
 
 Writing your own plugin
-~~~~~~~~~~~~~~~~~~~~~~~
+-----------------------
 
 .. note:: The Certbot team is not currently accepting any new DNS plugins
     because we want to rethink our approach to the challenge and resolve some
@@ -338,17 +338,69 @@ it was not installed properly.
 Once you've finished your plugin and published it, you can have your
 users install it system-wide with `pip install`. Note that this will
 only work for users who have Certbot installed from OS packages or via
-pip. Users who run `certbot-auto` are currently unable to use third-party
-plugins. It's technically possible to install third-party plugins into
-the virtualenv used by `certbot-auto`, but they will be wiped away when
-`certbot-auto` upgrades. If you'd like your plugin to be used alongside
-the Certbot snap, you will also have to publish your plugin as a snap.
-Certbot's DNS plugins and the README file in ``tools/snap/`` provide a
-starting reference for how to do this.
+pip.
 
 .. _`setuptools entry points`:
     https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points
 
+Writing your own plugin snap
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you'd like your plugin to be used alongside the Certbot snap, you
+will also have to publish your plugin as a snap. Plugin snaps are
+regular confined snaps, but normally do not provide any "apps"
+themselves. Plugin snaps export loadable Python modules to the Certbot
+snap.
+
+When the Certbot snap runs, it will use its version of Python and prefer
+Python modules contained in its own snap over modules contained in
+external snaps. This means that your snap doesn't have to contain things
+like an extra copy of Python, Certbot, or their dependencies, but also
+that if you need a different version of a dependency than is already
+installed in the Certbot snap, the Certbot snap will have to be updated.
+
+Certbot plugin snaps expose their Python modules to the Certbot snap via a
+`snap content interface`_ where ``certbot-1`` is the value for the ``content``
+attribute. The Certbot snap only uses this to find the names of connected
+plugin snaps and it expects to find the Python modules to be loaded under
+``lib/python3.8/site-packages/`` in the plugin snap. This location is the
+default when using the ``core20`` `base snap`_ and the `python snapcraft
+plugin`_.
+
+The Certbot snap also provides a separate content interface which
+you can use to get metadata about the Certbot snap using the ``content``
+identifier ``metadata-1``.
+
+The script used to generate the snapcraft.yaml files for our own externally
+snapped plugins can be found at
+https://github.com/certbot/certbot/blob/master/tools/snap/generate_dnsplugins_snapcraft.sh.
+
+Once you have created your own snap, if you have the snap file locally,
+it can be installed for use with Certbot by running:
+
+.. code-block:: shell
+
+    snap install --classic certbot
+    snap set certbot trust-plugin-with-root=ok
+    snap install --dangerous your-snap-filename.snap
+    sudo snap connect certbot:plugin your-snap-name
+    sudo /snap/bin/certbot plugins
+
+If everything worked, the last command should list your plugin in the
+list of plugins found by Certbot. Once your snap is published to the
+snap store, it will be installable through the name of the snap on the
+snap store without the ``--dangerous`` flag. If you are also using
+Certbot's metadata interface, you can run ``sudo snap connect
+your-snap-name:your-plug-name-for-metadata certbot:certbot-metadata`` to
+connect your snap to it.
+
+.. _`snap content interface`:
+    https://snapcraft.io/docs/content-interface
+.. _`base snap`:
+    https://snapcraft.io/docs/base-snaps
+.. _`python snapcraft plugin`:
+    https://snapcraft.io/docs/python-plugin
+
 .. _coding-style:
 
 Coding style