DOC: Restructure top level sectioning

[VeckoTheGecko]

This PR proposes a restructure to the documentation at the top level

Previous:

Home
quickstart
tutorials
contributing
OceanParcels <https://oceanparcels.org>

Now:

Home
user-guide
api
contributing
OceanParcels <https://oceanparcels.org>

This new top-level structure creates a User Guide section where the Quickstart and Tutorial pages live. With the old structure, it wasn't clear where user content would go that isn't quickstart or tutorials (e.g., info on how 'fetch' works).

This is also useful for #105 where these top level items occupy the navbar, hence we can't have too many.

I think this restructure is best to do now rather than later (as this would break documentation links that have been shared before).

[erikvansebille]

I'd like to see the new page after #105 is merged in, but for now I also think there could be a bit more/thorough introduction text at the top of the landing page. @ammedd, could you write a paragraph that better fleshes out what the project/tool is about? Perhaps also highlighting the difference between the VirtualShipParcels tool and the Virtual Ship Classroom material?

[VeckoTheGecko]

I also think there could be a bit more/thorough introduction text at the top of the landing page ... write a paragraph that better fleshes out what the project/tool is about?

This can be done by modifying the README, as the landing page is just a copy of the readme (trimming off some of the top). Anything after <!-- SPHINX-START --> will be included on the landing page.

[VeckoTheGecko]

I'd like to see the new page after #105 is merged in

Done @erikvansebille See docs