Feature/add external docs config option

[0xCAB0]

• Change the default function prompt to make the AI generate always markdown and when it is not external_docs mode add @doc syntax to the string.
• Update def docs_to_module_doc_node to save the output to a file at lazy_doc/{module/path/filename.md as follows:

defmodule LazyDoc.Providers.GithubAI do
    @doc File.read!("lazy_doc/lazy_doc/providers/github_ai/request_promt.md")
    def request_prompt(prompt, model, token, params \\ []) do
     req_query(prompt, model, token, params)
     |> Req.post()
  end
end

[pxp9]

lgtm