[PROPOSAL] Hint to add :ex_doc dependency when running `mix docs` and docs task not present #8554

[eksperimental]

Hi everyone,

I was recently testing ex_doc and tinkering with a new Elixir project, and I stumbled upon this:

When running mix docs in a project with no docs task, we should special case and
give a suggestion saying:

**************
If you want to build the docs for this project, you need to add ex_doc as your dependency,
please add {:ex_doc, only: :dev, runtime: false}, to your deps/0 functions in your mix.exs file,
so it looks like this:

defp deps do
[
{:ex_doc, only: :dev, runtime: false},
]
end

and run mix deps.get and mix docs

**************

before working on a PR, I would like to know if there's interest in such a feature
thank you for your feedback,
- Eksperimental