Use mamba to speed up conda

https://docs.readthedocs.io/en/stable/guides/conda.html#making-builds-faster-with-mamba

Author: raucao

.readthedocs.yml

@@ -1,4 +1,9 @@
 version: 2
 
+build:
+  os: "ubuntu-20.04"
+  tools:
+    python: "mambaforge-4.10"
+
 conda:
   environment: doc/environment.yml