Documentation dependencies (#2252)

* Use a trailing slash
recommended by mkdocs-material

* Update doc requirements
Let mkdocs-material resolve its imaging dependencies (cairosvg and
pillow) and fix a dependabot warning along the way.

Reference compatible versions by major.minor.

docs/requirements.txt

@@ -1,7 +1,5 @@
-cairosvg~=2.7.1
-mkdocs-include-markdown-plugin~=6.2.2
-mkdocs-macros-plugin~=1.2.0
-mkdocs-material~=9.5.18
-mkdocs-minify-plugin~=0.7.1
-mkdocs-redirects~=1.2.1
-pillow~=10.1.0
+mkdocs-include-markdown-plugin~=7.1
+mkdocs-macros-plugin~=1.3
+mkdocs-material[imaging]~=9.5
+mkdocs-minify-plugin~=0.7
+mkdocs-redirects~=1.2