Add more docs examples for offset encoding

It would be nice if we could add a page about the new offset encoding that contained common examples. In addition to the offset barplot in the VegaLite docs, I think these examples would be nice:

import altair as alt
from vega_datasets import data

source = data.barley.url

alt.Chart(source).mark_point().encode(
    x='site:O',
    y='yield:Q',
    xOffset='year:N',
    color='year:N',
)

alt.Chart(source).mark_point().encode(
    x='site:O',
    y='yield:Q',
    xOffset=alt.XOffset('offset:Q', scale=alt.Scale(domain=(0, 3)))
).transform_calculate(
    offset='random()'
)

alt.Chart(source, width=alt.Step(40)).mark_point().encode(
    x='site:O',
    y='yield:Q',
    xOffset=alt.XOffset('offset:Q', scale=alt.Scale(domain=(0, 10))),
    color='year:N'
).transform_calculate(
    offset="datum.year == 1932 ? 5 + random() : 0 + random()"
)

[binste]

Just so you are aware, I will work on the encoding pages next, expanding the current one and potentially adding separate ones for certain concepts similar to the vega-lite docs. I will also clarify the distinction between value and datum as discussed in #2572

[ChristopherDavisUCI]

In case it’s helpful, here are two short offset examples that I put together a few months ago: https://github.com/ChristopherDavisUCI/altair/commit/af0c3b4c1d27c366fb66fb6743aa15d5d30a56ce