fix(docs): add plantuml as a dependency

This wasn't listed on the web interface configuration. Perhaps it came
preloaded. Anyway, it's needed for diagrams. Add it back.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I20c1eb0e8d5abaa3533169dd9704cbd3b0eb06a5