RTD: add a developer comment in .readthedocs.yml (aiidateam#6864)

When `uv` creates a virtual environment, it will not automatically activate it. 
However this is not a problem, since the subsequent commands in `.readthedocs.yml`, meaning  `uv sync` & `uv run` both pick up the created environment automatically.

This commit clarifies the behavior for developers.

---------

Co-authored-by: Daniel Hollas <[email protected]>

Author: khsrali

.readthedocs.yml

@@ -20,6 +20,8 @@ build:
     - asdf install uv 0.5.22
     - asdf global uv 0.5.22
     create_environment:
+    # Create a virtual environment in '.venv/' folder
+    # which is picked up automatically by `uv sync` and `uv run` commands below
     - uv venv
     install:
     - uv sync --extra docs --extra tests --extra rest --extra atomic_tools