docs: add mamba to RTD config

.readthedocs.yml

@@ -5,6 +5,13 @@
 # Required
 version: 2
 
+# Use mamba instead of conda
+build:
+  os: "ubuntu-20.04"
+  tools:
+    python: "mambaforge-4.10"
+
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
    configuration: docs/conf.py
@@ -14,7 +21,7 @@ sphinx:
 #   - pdf
 
 # Optionally set the version of Python and requirements required to build your docs
-mamba:
+conda:
   environment: docs/environment.yml
 
 # python: