docs: Include CSS snippet at build time

Author: pawamoy

docs/usage/customization.md

@@ -57,33 +57,7 @@ Here are some CSS rules for the
 [*Material for MkDocs*](https://squidfunk.github.io/mkdocs-material/) theme:
 
 ```css
-/* Indentation. */
-div.doc-contents:not(.first) {
-  padding-left: 25px;
-  border-left: .05rem solid var(--md-typeset-table-color);
-}
-
-/* Mark external links as such. */
-a.autorefs-external::after {
-  /* https://primer.style/octicons/arrow-up-right-24 */
-  background-image: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path fill="rgb(0, 0, 0)" d="M18.25 15.5a.75.75 0 00.75-.75v-9a.75.75 0 00-.75-.75h-9a.75.75 0 000 1.5h7.19L6.22 16.72a.75.75 0 101.06 1.06L17.5 7.56v7.19c0 .414.336.75.75.75z"></path></svg>');
-  content: ' ';
-
-  display: inline-block;
-  vertical-align: middle;
-  position: relative;
-  bottom: 0.1em;
-  margin-left: 0.2em;
-  margin-right: 0.1em;
-
-  height: 0.7em;
-  width: 0.7em;
-  border-radius: 100%;
-  background-color: var(--md-typeset-a-color);
-}
-a.autorefs-external:hover::after {
-  background-color: var(--md-accent-fg-color);
-}
+--8<-- "docs/css/mkdocstrings.css"
 ```
 
 ### Recommended style (ReadTheDocs)