Docs needs a quickstart

The "What is this" is quite a bit long to read and takes a while to run.

I recommend showing very simple example with no deps:
```python
import xarray as xr
import xarray_sql as xql

ds = xr.tutorial.open_dataset('air_temperature')
ctx = xql.XarrayContext()
ctx.from_dataset('air', ds, chunks=dict(time=10))
ctx.sql('SELECT MAX(air) FROM air')


# max(air.air)
# --
# 317.40000000000003
```
