Clean up user guide example code and docs (pt 2)

[neverett]

This PR continues the work in #1584:

• Moves user guide examples back into the examples directory so those examples can continue to be serialized and packaged for release, even though they are no longer being used to produce documentation.
• Updates the custom auto_examples.py extension to exclude user guide examples from being processed and turned into Markdown files, since the user guide Markdown files now live in the flyte repo.
• Removes "Getting Started" Markdown files that have been moved to the flyte repo and updates contributing guidelines to clarify where and how to contribute code and docs.

Note: Merge this PR before merging flyteorg/flyte#5207

[cosmicBboy]

@neverett @ppiegaze btw these will be interpreted as external links by sphinx and will open a new tab when you click on these links (see flyteorg/flyte#5159). Can we use the actual path so that sphinx understands these as internal links?

[cosmicBboy]

An example of this behavior is the ChatGPT agent link in https://docs.flyte.org/en/latest/deployment/agents/chatgpt.html#deployment-agent-setup-chatgpt

[neverett]

I think it's okay if there are external links in the index file in this repo since the actual index file for the docs site lives in the flyte repo. Anyone who builds flytesnacks docs locally should only be doing so to check specific integrations and/or tutorials pages, since almost all other docs live in flyte.

The ChatGPT agent link in https://docs.flyte.org/en/latest/deployment/agents/chatgpt.html#deployment-agent-setup-chatgpt needs to be fixed in the flyte repo, which I can take care of.