Docs: UI extensions - provide guidance on required properties in plugin.json

[sympatheticmoose]

https://grafana.com/developers/plugin-tools/reference/ui-extensions-reference/ui-extensions#addlink addLink has a bunch of required/optional properties listed here

We have guidance here https://grafana.com/developers/plugin-tools/how-to-guides/ui-extensions/register-an-extension#update-the-pluginjson-metadata, but it isn't very explicit in terms of which properties are required or ignored in the plugin.json - it could be interpreted just as an example

Came up in a conversation regarding the icon property

[sympatheticmoose]

cc @leventebalogh based on our chat

[leventebalogh]

I had a short chat about this with @hugohaggmark, and I feel that it probably needs a bit more clarification on how we should do it - and as I am out for quite some time from today I didn't open a PR yet.

For me the main concern is how clear it is going to be for devs if we add another column here (as it's a table for the parameters of the addLink() function), even though it's closer to the plugin.json schema then to the runtime API definition.