Is your feature request related to a problem? Please describe.
If there's an OpenAPI 3 JSON specification available, I can automatically generate part of a Wallabag client (or even an API compatible server!)

It's supported by the Nelmio library you're using, but not turned on.

Describe the solution you'd like
A clear and concise description of what you want to happen.
It looks like https://symfony.com/bundles/NelmioApiDocBundle/current/index.html#installation says that you need something as "simple" as

app.swagger:
    path: /api/doc.json
    methods: GET
    defaults: { _controller: nelmio_api_doc.controller.swagger }

next to https://github.com/wallabag/wallabag/blob/master/app/config/routing.yml#L5 but I don't know how to do it with the syntax you're using.

Describe alternatives you've considered
Learning enough about Symphony and hacking routing.yaml myself