Add optionnal simple telemetry so we now how many docs instances are out there

[virgile-dev]

We recently added a table of know instances on the README.
People are encouraged to add theirs via a PR.
Ideally we'd love to be able to know how many instances are out there without having to ask people to do a PR.
The way I see it Docs instances could send a ping and a couple info (Docs version, Number of users, of Docs).
This ping would be optional and could be disabled by setting and env variable to false