Stylize Read the Docs as per guidelines (#579)

Author: pradyunsg

src/sphinx_book_theme/assets/styles/extensions/_readthedocs.scss

@@ -47,7 +47,7 @@
         margin-right: auto;
 
         &:after {
-          content: "ReadTheDocs";
+          content: "Read the Docs";
           font-family: sans-serif;
           font-weight: bold;
           margin-left: 0.2em;