Docs: update conda config key to mention build.tools.python (#10672)

Make it clear that when using `conda` it's required to use `build.tools.python`
with Conda or Mamba.

Related #8595 (comment)

Author: humitos

docs/user/config-file/v2.rst

@@ -236,6 +236,11 @@ Configuration for Conda support.
 
    version: 2
 
+   build:
+     os: "ubuntu-22.04"
+     tools:
+       python: "mambaforge-22.9"
+
    conda:
      environment: environment.yml
 
@@ -247,6 +252,10 @@ The path to the Conda `environment file <https://conda.io/projects/conda/en/late
 :Type: ``path``
 :Required: ``false``
 
+.. note::
+
+   When using Conda, it's required to specify ``build.tools.python`` to tell Read the Docs to use whether Conda or Mamba to create the environment.
+
 build
 ~~~~~