custom plugin manager documentation

[psifertex]

With the latest changes on dev the docs at https://docs.binary.ninja/dev/plugins.html#using-your-own-plugin-repository are incorrect. We need to fix it to point to the new v2 schema and adjust any other docs as required.

[psifertex]

Current schema is only available via https://staging.extensions.binary.ninja/v2/schema for now, but should be live on the main extension site in a bit. We'll need to update the docs to refer to that and potentially even include a minimal example.