Add info about new `Plotly` methods to reference page

[etpinard]

In the past few weeks, three new Plotly methods were added to plotly.js. We should add info about them on /javascript/plotlyjs-function-reference/.

These new methods are:

• Plotly.purge in v1.7.0 which clears the graph div to its pre-Plotly.plot state
• Plotly.toImage in v1.11.0 which turns an existing plotly graph into an image string
• Plotly.downloadImage in v1.11.0 which turns programatically download an existing plotly graph.

[timelyportfolio]

Here is a codepen example of toImage that could be adapted for documentation.

[yankev]

Just added these. Let me know if there are any changes I should make.

[cldougl]

@yankev I would remove the svg example from https://plot.ly/javascript/plotlyjs-function-reference/#generating-image-strings-with-plotlytoimage I think having the png and pdf (pro example) is enough -- the svg just results in a large text block so it's not a super great visual

everything else is 👍 and you can close this issue after the update ^

[yankev]

@cldougl, k removed. Closing #395