Improve syntax highlighting in MkDocs pages

The syntax of PHP code was not showing with different colors, but shown
as plain/text.

.readthedocs.yaml

@@ -5,5 +5,9 @@ build:
   tools:
     python: "3.12"
 
+python:
+  install:
+    - requirements: docs/requirements.txt
+
 mkdocs:
   configuration: mkdocs.yml

docs/requirements.txt

@@ -0,0 +1,3 @@
+mkdocs>=1.5.0
+pymdown-extensions>=10.0
+

mkdocs.yml

@@ -1,2 +1,10 @@
 site_name: Respect\Validation
 theme: readthedocs
+
+markdown_extensions:
+  - pymdownx.highlight:
+      anchor_linenums: true
+      line_spans: __span
+      pygments_lang_class: true
+  - pymdownx.inlinehilite
+  - pymdownx.superfences