Provide an automatically generated table of content for selected topics

[tanius]

For some posts in Discourse, especially our manual topics, having an automatically generated table of contents would be great to get around maintaining it manually like we do now.

Options:

1. A custom plugin that generates it in the rendering from a BBCode element placed into the post, such as [toc levels=2].

2. A table of contents in the sidebar, using the existing plugin employed for example here.