Following art #30743, there is no longer any Tuleap Plugins outside the main source tree.

The Plugin development documentation was on https://docs.tuleap.org to make it more accessible for out of tree developments but has always been a PITA to maintain and to keep in sync.

Having the dev doc in Tuleap sources will help us to do a better job at doc maintenance and have a more clear cut between Tuleap App (3rd party development based on public APIs) and Tuleap Plugins.