Use Mamba in place of Conda

As per <https://docs.readthedocs.io/en/stable/guides/conda.html>

Use Mamba in place of Conda
9704ed42 · Ben Mares · 87fb6262 · 9704ed42
--- a/readthedocs.yml
+++ b/readthedocs.yml
@@ -3,3 +3,7 @@ sphinx:
  configuration: doc/conf.py
 conda:
  environment: doc/environment.yml
+build:
+  os: "ubuntu-20.04"
+  tools:
+    python: "mambaforge-4.10"