docs: use doxygen-awesome-css (#2758)

.readthedocs.yaml

@@ -10,9 +10,8 @@ version: 2
 build:
   os: ubuntu-24.04
   tools:
-    python: "3.11"
+    python: "miniconda-latest"
   apt_packages:
-    - graphviz  # required to build diagrams
     - libboost-locale-dev  # required for rstcheck in cpp code block
   jobs:
     post_build:
@@ -20,6 +19,11 @@ build:
       - rstcheck -r .  # lint rst files
       # - rstfmt --check --diff -w 120 .  # check rst formatting
 
+# using conda, we can get newer doxygen and graphviz than ubuntu provide
+# https://github.com/readthedocs/readthedocs.org/issues/8151#issuecomment-890359661
+conda:
+  environment: docs/environment.yml
+
 # submodules required for include statements
 submodules:
   include: all