Update docs WRT relaxed API token requirements

As @pombredanne confirmed in a private chat, https://public.vulnerablecode.io/ can now also be queried without an API token, although the [docs](https://vulnerablecode.readthedocs.io/en/latest/api.html#how-to-use-openapi-documentation) still say that "To use the endpoints you *need* to authenticate with an API key".

The docs should be updated in this regard, and the implications of not using an API token (like any rate limit being applied) should be mentioned as well.