when we document `kolibri plugin enable | disable`, ensure that PEX has special note

[indirectlylit]

(original issue was transfered from Kolibri User Docs)

In order to install and use externally-built plugins with PEX, the environment variable PEX_INHERIT_PATH=1 must be set

[radinamatic]

From the original issue:

We don't publicly document plugin enable/disable functionality because it's not stable yet, but when we do we'll need to add this as a note.

@indirectlylit 2 questions:

1. We never covered anything related to using externally-built plugins in the user documentation. Which plugin would be a good example to add?
2. Is this already stable enough to be added to the docs?

[indirectlylit]

that's a good point. I'll transfer this to Kolibri internal dev docs