Consider a recommended name for OPA server configuration file

[anderseknert]

The OPA docs provide no suggestions on what to name the OPA server configuration file. While this may not matter to OPA, as configuration must be provided explicitly via the --config-file flag, there are other tools for which it would be helpful to be able to detect and OPA configuration file merely by it's filename. One example of this would be the JSON schema store, where we could publish the JSON schema for the OPA configuration file format. But the catalog expects a fileMatch property to determine if a file is an OPA configuration file or not. Another example would be Regal, where we might want to lint OPA configurations as part of linting a project or workspace.

I guess it could also be considered if opa run --server . automatically should pick up a config file given one with the well-known name exists. But that can be a later concern. For now, I'm suggesting we update the docs to suggest a name for an OPA configuration file, and explain that while it's not mandatory, it can provide benefits for tooling, and from that the overall development experience.

As for the name, opa-conf.yaml is used by Styra DAS, so it has some adoption already. But it wouldn't necessarily have to be a single name. If there are other suggestions for names or even patterns, we can consider recommending a few of them as "well-known".