Replace remark-toc with Docusaurus-native generated index pages

[ptgott]

We use the custom remark-toc plugin (gravitational/docs) to generate a list of docs pages at the same directory level as the page that invokes the plugin.

To invoke the plugin, a page includes the following line:

(!toc!)

In Docusaurus, you can embed an auto-generated list of category items using the <DocCardList /> tag (see the Docusaurus docs).

Let's consider using this approach to reduce the code we need to maintain.