Add a troubeshooting doc

docs/diagnosing-dev-env-issues.md

+# Diagnosing Development Environment Issues
+
+If you're struggling to diagnose problems in your development environment, here are some tips which may help.
+
+## Python Problems
+
+1. Stear clear of any `asdf`/`mise` plugins which modify the `PYTHONPATH` environment variable. 
+   Known offenders include `asdf-checkov`. 
+   These plugins can be replaced with a `pip install`.
+
+1. On most projects, `direnv` will automatically configure a Python virtual environment for the project, 
+   using the `layout python3` command in `.envrc`. 
+   If your project does not do this and you are managing your own virtual environment, 
+   it's highly recommended that you reconsider and switch to using `direnv` for this. 
+
+1. If you're having troubles with `python`, consider running `rm -rf .direnv/python*` to clean up your virtual environment.
+
+    ```console
+    $ # delete the virtual environment and start again:
+    $ rm -rf .direnv/python*
+    $ # reload direnv to recreate your virtual environment
+    $ direnv reload
+    $ # reinstall python dependencies
+    $ ./scripts/prepare-dev-env.sh
+    ```
+
+    
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