split documentation notebooks by topic

[jmoralez]

Description

Splits the nbs/docs folder into:

• nbs/docs/getting-started
• nbs/docs/how-to-guides
• nbs/docs/tutorials Also splits the installation for distributed by having dask, ray and spark extras, instead of the single distributed extra that installed everything and updates the instructions accordingly.

Checklist:

• [x] This PR has a meaningful title and a clear description.
• [x] The tests pass.
• [x] All linting tasks pass.
• [x] The notebooks are clean.

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