Local extensions are not installed

Hi, first of all thanks for making this awesome CMS! So I am trying to write an extension, and the documentation says:

> in any Bolt site you can put local extensions within extensions/local/{author_name}/{extension_name}

(this is at https://docs.bolt.cm/3.6/extensions/basics/testing#phase-1-develop-your-extension-within-a-bolt-project)

However, there is probably something else to do, because Bolt does not seem to pick up the extension (it does not appear as an installed extension on the extensions page, and calling extension methods fail).

I did change the minimum-stability to dev in config.yml

And to exclude a problem with the extension itself, I downloaded a known-working extensions from github (https://github.com/designspike/bolt-extension-menuchain) and I renamed the directories so that the extension is in extensions/local/designspike/menuchain, as suggested in the docs above:

gpothier@tadzim4:~/myboltroot$ ls extensions/local/designspike/menuchain/
composer.json README.md src

The documentation doesn't mention any additional step for this to work. What am I missing?