documentation on spatial data (#2750)

* add section for data dict

* define index page for data section

* infrastructure other pages

* docs spatial data

* empty non spatial pages

* Apply suggestions from code review

Love the review, thanks @joelostblom!

Co-authored-by: Joel Ostblom <joelostblom@users.noreply.github.com>

* change geojson url to a dataset with continents
include mesh option for topojson
fix few typos

* fix reference

* end with __geo_interface__

Co-authored-by: Joel Ostblom <joelostblom@users.noreply.github.com>

doc/user_guide/data.rst

@@ -441,6 +441,7 @@ data before usage in Altair using GeoPandas for example as such:
    :hidden:
 
    self
+   data/index
    encodings/index
    marks/index
    transform/index

doc/user_guide/data/dataframe.rst

@@ -0,0 +1,8 @@
+.. currentmodule:: altair
+
+.. _user-guide-dataframe-data:
+
+DataFrame
+~~~~~~~~~
+
+Describe

doc/user_guide/data/dict.rst

@@ -0,0 +1,8 @@
+.. currentmodule:: altair
+
+.. _user-guide-dict-data:
+
+Dictionary
+~~~~~~~~~~
+
+Describe

doc/user_guide/data/generator.rst

@@ -0,0 +1,8 @@
+.. currentmodule:: altair
+
+.. _user-guide-generator-data:
+
+Generated data
+~~~~~~~~~~~~~~
+
+Describe

doc/user_guide/data/index.rst

@@ -0,0 +1,58 @@
+.. currentmodule:: altair
+
+.. _user-guide-data:
+
+Data
+~~~~
+
+The basic data model used by Altair is tabular data,
+similar to a spreadsheet or database table.
+Individual datasets are assumed to contain a collection of records (rows),
+which may contain any number of named data fields (columns).
+Each top-level chart object (i.e. :class:`Chart`, :class:`LayerChart`,
+:class:`VConcatChart`, :class:`HConcatChart`, :class:`RepeatChart`,
+and :class:`FacetChart`) accepts a dataset as its first argument.
+
+While the most common way to provide Altair with a dataset is via a pandas DataFrame,
+there are many different ways of specifying a dataset:
+
+=========================================  ================================================================================
+Data                                       Description
+=========================================  ================================================================================
+:ref:`user-guide-dataframe-data`           A `pandas DataFrame <http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.html>`_.
+:ref:`user-guide-dict-data`                A :class:`Data` or related object (i.e. :class:`UrlData`, :class:`InlineData`, :class:`NamedData`).                                 
+:ref:`user-guide-url-data`                 A url string pointing to a ``json`` or ``csv`` formatted text file.                                       
+:ref:`user-guide-spatial-data`             A `geopandas GeoDataFrame <http://geopandas.org/data_structures.html#geodataframe>`_, `Shapely Geometries <https://shapely.readthedocs.io/en/latest/manual.html#geometric-objects>`_, `GeoJSON Objects <https://github.com/jazzband/geojson#geojson-objects>`_ or other objects that support the ``__geo_interface__``.
+:ref:`user-guide-generator-data`           A generated dataset such as numerical sequences or geographic reference elements.
+=========================================  ================================================================================
+
+When data is specified as a DataFrame, the encoding is quite simple, as Altair
+uses the data type information provided by pandas to automatically determine
+the data types required in the encoding. For example, here we specify data via a pandas DataFrame
+and Altair automatically detects that the x-column should be visualized on a quantitative scale
+and that the y-column should be visualized on a categorical scale:
+
+.. altair-plot::
+
+   import altair as alt
+   import pandas as pd
+
+   data = pd.DataFrame({'x': ['A', 'B', 'C', 'D', 'E'],
+                        'y': [5, 3, 6, 7, 2]})
+   alt.Chart(data).mark_bar().encode(
+       x='x',
+       y='y',
+   )
+
+If we are not happy with the scale that Altair chooses for visualizing that data,
+we can change it by either changing the data types in the underlying pandas dataframe,
+or by changing the :ref:`encoding-data-types` in Altair.
+
+.. toctree::
+   :hidden:
+
+   dataframe
+   dict   
+   url
+   spatial
+   generator