Update ambiant install docs to specify Python version

I also modified the overall intent of the instructions slightly, so
that they direct the user to set up a new Conda environment. The old
instructions were oriented more towards adding Nextstrain to an
existing Conda enviroment, which (IMO) seems like a less common
scenario, and also one more likely to be undertaken by a more
experienced user who will be able to adapt these instructions on the
fly.

src/install.rst

@@ -213,7 +213,7 @@ Set up a Nextstrain runtime
 
       The ambient runtime does not require a particular setup method; it will work as long as the programs you wish to use are available.
 
-      The following describes how to accomplish this using a custom Conda environment as an example. You should be familiar with the `basics of Conda <https://conda.io/projects/conda/en/latest/user-guide/getting-started.html>`__ before proceeding.
+      The following describes how to accomplish this by creating a new custom Conda environment, as an example. You should be familiar with the `basics of Conda <https://conda.io/projects/conda/en/latest/user-guide/getting-started.html>`__ before proceeding. If you want to add Nextstrain to an existing Conda environment, please make sure you're using a compatible Python version (we suggest v3.10.13) and activate that environment instead of creating a new one.
 
       .. tabs::
 

src/snippets/ambient-setup.rst

@@ -1,8 +1,11 @@
-1. Activate a Conda environment that you wish to use:
+1. Create a new Conda environment and install Python 3.10.13:
 
    .. code-block:: bash
 
+      conda create -n <your-environment-name>
       conda activate <your-environment-name>
+      conda install --override-channels --strict-channel-priority \
+            -c conda-forge --yes python=3.10.13
 
 2. Install all the necessary software: