Update documentation

Author: humitos

docs/development/docs.rst

@@ -31,14 +31,14 @@ you may want to verify those changes locally before pushing upstream.
 
       $ git clone --recurse-submodules https://github.com/readthedocs/readthedocs.org/
 
-#. create a virtual environment with Python 3.6
-   (preferably the latest release, 3.6.15 at the time of writing),
+#. create a virtual environment with Python 3.8
+   (preferably the latest release, 3.8.12 at the time of writing),
    activate it, and upgrade both pip and setuptools:
 
    .. code-block:: console
 
       $ cd readthedocs.org
-      $ python3.6 -m venv .venv
+      $ python3.8 -m venv .venv
       $ source .venv/bin/activate
       (.venv) $ python -m pip install -U pip setuptools
 

docs/development/install.rst

@@ -142,7 +142,7 @@ save some work while typing docker compose commands. This section explains these
 ``inv docker.test``
     Runs all the test suites inside the container.
 
-    * ``--arguments`` will pass arguments to Tox command (e.g. ``--arguments "-e py36 -- -k test_api"``)
+    * ``--arguments`` will pass arguments to Tox command (e.g. ``--arguments "-e py38 -- -k test_api"``)
 
 ``inv docker.pull``
     Downloads and tags all the Docker images required for builders.

docs/development/tests.rst

@@ -46,13 +46,13 @@ To target a specific environment:
 
 .. prompt:: bash
 
-    tox -e py36
+    tox -e py38
 
 The ``tox`` configuration has the following environments configured. You can
 target a single environment to limit the test suite:
 
-py36
-    Run our test suite using Python 3.6
+py38
+    Run our test suite using Python 3.8
 
 lint
     Run code linting using `Prospector`_. This currently runs `pylint`_,