Docs: Clarify Extending Core Plugin Config Functions

### Is your feature related to a problem?

The existing documentation is not clear on which plugins have a `require("plugins.configs.X")`. One might assume that all plugins inside astronvim have them but they don't. Which means the plugins that don't have this configuration must be set in a different way.

example:

If you want to set telescope you can follow the example in the [official doc](https://astronvim.com/Recipes/custom_plugins#extending-core-plugin-config-functions) but if you want to setup null-ls sources the user_example configuration suggests a different approach https://github.com/AstroNvim/user_example/blob/main/plugins/null-ls.lua

It is not clear to me what happens with the `config` object returned in the example and how is that related to the [code that sets null-ls](https://github.com/AstroNvim/AstroNvim/blob/291c43b0bdd551ebaf4de11e4f41ff7ddb38559a/lua/plugins/lsp.lua#L30) in astronvim.

In general terms there are really two different ways to extend core plugins and they are not clearly documented.

### Describe the new feature

* A list (or link) pointing to the plugins that have a `plugins.configs` defined and should follow the require pattern
* An additional examples for plugins that don't have their own `plugin.configs` and should follow the `opts` function pattern.

I understand that you point to the lazy.nvim documentation but the link is to a general readme file and it is hard to know how much of the work is done by lazy.nvim and how much by astronvim code which ends up on multiple trial and error to see what works.

### Additional context

Thanks for such a nice work in Astronvim!