Docs: Update CSS Guidelines to align with new shared config

- also make changes to set up similar sections to JS docs

docs/contributing/css_guidelines.rst

@@ -1,7 +1,7 @@
 CSS coding guidelines
 ===========================
 
-Our CSS is written in Sass, using the SCSS syntax.
+Our CSS is written in `Sass <https://sass-lang.com/>`_, using the SCSS syntax.
 
 Compiling
 ~~~~~~~~~
@@ -32,13 +32,16 @@ automatically recompiling when any changes are observed, by running:
     $ npm start
 
 
-Linting SCSS
+Linting and formatting SCSS
-~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Wagtail uses the `stylelint <https://stylelint.io/>`_ linter.
 You'll need Node.js and npm on your development machine.
 Ensure project dependencies are installed by running ``npm install --no-save``
 
+Linting code
+------------
+
 Run the linter from the wagtail project root:
 
 .. code-block:: console
@@ -49,12 +52,27 @@ Run the linter from the wagtail project root:
 The linter is configured to check your code for adherance to the guidelines
 below, plus a little more.
 
+Formatting code
+---------------
+
 If you want to autofix errors, you can run that command directly with:
 
 .. code-block:: console
 
     $ npm run lint:css -- --fix
 
+Changing the linter configuration
+---------------------------------
+
+To edit ignored files, add an entry to the ``.stylelintrc.yaml`` file under ``ignoreFiles``.
+
+To the configuration for the linting rules is managed in an external
+repository so that it can be easily shared across other Wagtail projects
+or plugins. This configuration can be found at
+`stylelint-config-wagtail <https://github.com/wagtail/stylelint-config-wagtail>`_.
+
+Styleguide Reference
+~~~~~~~~~~~~~~~~~~~~
 
 Spacing
 -------