"External dependencies" doc page should mention and link to dry-initializer

There's no information on this page about how exactly the `option` DSL works. We could clarify things with a sentence saying it is provided by dry-initializer and linking to its docs.

That would help people e.g. wanting to work out how to provide default values for options, etc., without us needing to repeat that information here.

Open the “External dependencies” documentation page and read the section describing the `option` DSL. Add a sentence identifying dry-initializer as the provider and link to its documentation; the work is done when readers can follow that link for details such as default option values.

Written by the indexing model from the issue text.