aspect ratio docs and backlinks

[chriddyp]

From plotly/plotly.js#1522

docs for:

• r
• python
• javascript

After we have docs, we can close and comment on:

• way to preserve square aspect ratio plotly.py#70
• https://community.plot.ly/t/aspect-ratio-plotly/3215
• Request: Add change aspect ratio plotly.py#285

cc @cldougl

[cldougl]

waiting for https://github.com/plotly/streambed/pull/9387