Add timeline and cards plugins

[wwarriner]

Plugins: https://github.com/Neoteroi/mkdocs-plugins

Use-case for timeline is to communicate upcoming events, maintenance, etc, while keeping a rolling timeline. This directly applies to communication for our ongoing data center migration.

Use-case for cards is to quickly guide readers to important and/or popular topics that are otherwise deeply nested in our documentation. These are more compact than lists and other purely-textual information. This directly applies to the front page, and may also work on other index pages.