Docs: Change formatting of paragraphs under <h2> on chapter pages

We'd like to put explanation on chapter pages. Reading large amounts of
text that is center justified feels strange. This change does not modify
the format of existing chapter pages. However, any <p> element after a
<h2> element on a chapter page will now be formatted consistently with
the rest of the documentation.

docs/config/_default/config.toml

@@ -38,7 +38,7 @@ theme = "learn"
   # Order sections in menu by "weight" or "title". Default to "weight"
   ordersectionsby = "weight"
   # Provide a list of custom css files to load relative from the `static/` folder in the site root.
-  custom_css = ["css/labels.css", "css/shadow-around-images.css", "css/table-of-contents.css"]
+  custom_css = ["css/chapters.css", "css/labels.css", "css/shadow-around-images.css", "css/table-of-contents.css"]
 
 [outputs]
   home = ["HTML", "RSS", "JSON"]

docs/static/css/chapters.css

@@ -0,0 +1,4 @@
+#chapter h2 ~ p {
+    text-align: left;
+    font-size: 1rem;
+}