docs: use mamba to create doc env

Documentation build fails due to excessive memory usage.
Potential solution could be using mamba
instead of conda, the former is less memory intensive.

.readthedocs.yml

@@ -14,7 +14,7 @@ sphinx:
 #   - pdf
 
 # Optionally set the version of Python and requirements required to build your docs
-conda:
+mamba:
   environment: docs/environment.yml
 
 # python: