Docs: do not install as Sphinx extension

Closes #1222

docs/installing.rst

@@ -10,15 +10,10 @@ Install the ``sphinx_rtd_theme`` package (or add it to your ``requirements.txt``
 
  $ pip install sphinx_rtd_theme
 
-In your Sphinx project's ``conf.py`` file, add ``sphinx_rtd_theme`` to the list of enabled extensions and as the active theme:
+In your Sphinx project's ``conf.py`` file, define ``html_theme`` to activate the theme:
 
 .. code:: python
 
- extensions = [
- ...
- 'sphinx_rtd_theme',
- ]
-
  html_theme = "sphinx_rtd_theme"
 
 .. seealso::
@@ -29,19 +24,6 @@ In your Sphinx project's ``conf.py`` file, add ``sphinx_rtd_theme`` to the list
  Officially Supported versions of Python, Sphinx, and other dependencies.
 
 
-.. note::
-
- Adding this theme as an extension is what enables localization of theme
- strings in your translated output. If these strings are not translated in
- your output, either we lack the localized strings for your locale, or you
- are using an old version of the theme.
-
- ..
- comment about this note: it's possibly not necessary to add the theme as an extension.
- Rather, this is an issue caused by setting html_theme_path.
- See: https://github.com/readthedocs/readthedocs.org/pull/9654
-
-
 .. _howto_upgrade:
 
 How to upgrade
@@ -55,6 +37,5 @@ Otherwise, pip will ignore pre-releases. Add for instance ``sphinx-rtd-theme==1.
 .. tip::
  We recommend that you pin the version of Sphinx that your project is built with.
  We won't release sphinx-rtd-theme without marking its compatibility with Sphinx. So if you do not pin ``sphinx-rtd-theme`` itself, you will always get the *latest compatible* release.
-
- More information is available in Read the Docs' documentation on :doc:`rtd:guides/reproducible-builds`.
 
+ More information is available in Read the Docs' documentation on :doc:`rtd:guides/reproducible-builds`.