Remove docs & improve README

[giograno]

After an internal discussion (mostly after feedback from @thrau), I decided to drop the sphinx docs and keep all the docs related to the SDK in our official user documentation.

One of the biggest pro of sphinx was the API reference generation from docstrings which we can still re-introduce at some point in the official docs, if we want. I'd argue that the reference becomes less important once we open-source this repo.

In the context of this PR, I also improved our README, with a pointer to the docs for a quickstart and code samples.

[alexrashed]

After an internal discussion (mostly after feedback from @thrau), I decided to drop the sphinx docs and keep all the docs related to the SDK in our official user documentation.
One of the biggest pro of sphinx was the API reference generation from docstrings which we can still re-introduce at some point in the official docs, if we want. I'd argue that the reference becomes less important once we open-source this repo.

I think this is a very interesting discussion to have. It's totally fine to take them down for now, but it would be super interesting what the reasoning against publishing their own fine-grained docs is (in the PR description you only mentioned the "pro" of Sphinx).

I found a small issue in the README (wrong badge) which would be good to fix before merging, the rest of my comments really are just nitpicks. These shouldn't be blocking the merge (and are quite subjective I would say).

[giograno]

@alexrashed I was convinced by our internal discussion (especially by the comment of @thrau).
I was very attracted by the automatic generation of the API reference from docstrings but:

• we can still do this in the future in our main docs;
• getting started guides and code samples are more important for adoptions (and we have these already in the main docs).