documentation on spatial data

[mattijn]

Fix #2740.

This PR introduces a different section how to specify data.

The following sections are defined (each on its own page):

• DataFrame data
• Dictionary data
• URL data
• Spatial data
• Generator data

I only looked carefully to the page on spatial data, the others are merely infrastructure code that can be extended and improved by other PRs.

[joelostblom]

Thanks for adding this section! I have some minor comments on wording and adding some links to other doc pages. I saw that you initially added the other data pages as well but then removed them, do you think we should add that (and remove the "specifying data" page) in a separate PR or add it to this one?

[mattijn]

I saw that you initially added the other data pages as well but then removed them, do you think we should add that (and remove the "specifying data" page) in a separate PR or add it to this one?

I removed them again since I'd just copy+paste them into the different pages from the existing https://altair-viz.github.io/user_guide/data.html page. I did not yet remove this page, so I though it might probably be better that someone does this in a separate PR.

Probably good to check as well if these pages needs to be modified or extended.

[joelostblom]

This looks great from what I can see! The Altair documentation has gotten SO much better in terms of geographic data with this and the mark_geoshape PR you made earlier. Thank you @mattijn ! Feel free to merge unless you were planning to add something else. I opened #2761 for the follow up data sections