[mkdocs] See if there are any missing pages and add redirects

[robertclaus]

Before publishing, see if there are any missing pages (pages currently on plotly.com/python and plotly.com/python-api-reference) and add redirects.

[daexs]

The only missing page left without redirects is tile-density-heatmaps.md, which does not seem to have a redirect on the live site either.