use mkdocs-gallery for example / tutorial documentation

[pmeier]

Per title. This PR removes the examples/ folder and static markdown examples / tutorials and replaces them with executable gallery scripts.

We are going to lose the following two examples for now:

• Local LLMs: this was an absolute bare bones example with no narration / text. Plus, it is not up to date anyway and thus cannot be run. We should re-add such an example in the future.
• S3 documents: this functionality was moved into ragna-aws anyway, so we don't need to keep the example.

For that we gain one more example to showcase streaming LLM responses.

[pmeier]

@pavithraes In smarie/mkdocs-gallery#90 I'm working on adding support for async galleries. With that, writing a gallery feels exactly like a notebook in the sense that we can use await and other async syntax without the need of wrapping everything in asyncio.run or the like.

I've switched this PR to the branch where I'm adding async galleries. Have a look at the script that is part of this PR and how it renders: https://ragna--89.org.readthedocs.build/en/89/generated/gallery/example_python_api/

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[pmeier]

Blocked by #316 and #317.

[pavithraes]

Thanks for this PR, @pmeier :)

Overall, this looks good. My main (non-blocking) comment in around consistency in narrative text. It'll be nice to use a standard style guide for all narrative elements. However, we can always update for this later, so we can merge for now. :)

Edit: I do see that the CI is failing with a DeprecationWarning - but seems like that needs to be resolved separately.