doc: Promote documention on how to use external plugins up one level

Currently the front page at https://buildstream.gitlab.io/buildstream/ doesn't make it easy to discover how to use external plugins.

There is an 'External plugins' heading which simply links to the bst-external documentation (which currently doesn't document how to use external plugins, although bst-external!14 (merged) would fix that).

By promoting the "Plugin Origins and Versions" item up a level, we cause it to appear in the table of contents on the front page making it easier to find.