Clean up and move user guide Python files

[neverett]

This PR:

• Removes the majority of comments from the Python files in user guide sections (advanced_composition, basics, customizing_dependencies, data_types_and_io, development_lifecycle, extending, productionizing, testing) since those comments have been turned into Markdown in the Flyte repo.
• Moves the user guide Python files to a new example_code directory so they don't get consumed by the docs build process.
• Updates the .flake config file to ignore E402 module level import not at top of file errors because many of the user guide Python files contain multiple examples with multiple imports at different places in the file. I think the reason these errors didn't come up when the files lived in the examples directory was because they were processed by the auto_examples custom extension and not linted as Python files.

After this PR is merged, I will replace the embedded code snippets in the user guide Markdown docs in the Flyte repo with rli (remoteliteralinclude) directives that reference the user guide Python files in this repo. (See flyteorg/flyte#5207).

[ppiegaze]

LGTM!