Add details on adding completion for core plugin configuration

[mehalter]

When configuring core plugins (AstroCore, AstroUI, AstorLSP), we have full Lua Language Server integration which allows documentation and auto completion while working on user configurations. This requires specifying the opts type like this:

{
  "AstroNvim/astrocore",
  ---@type AstroCoreOpts
  opts = {
    ...
  },
}

Similar for AstroUIOpts and AstroLSPOpts. We should add this detail to the documentation to make it easier for users to use this feature.

[mehalter]

Implemented on v4 branch!