By using custom Ecto types for params, they can be
serialized/deserialized by Ecto.

In particular, erlang terms are deserialized from the database.

Add custom type for JobQueue params

Before this change, EctoJob.JobQueue.params() was being incorrectly set
to `:map`, instead of the desired `map()`, which causes Dialyzer to
think the contract any `EctoJob.JobQueue` function whose `@spec` uses
the `params()` type.

bug: Fix params type definition with :map option

Besides other documentation changes, this commit ensures the generated
HTML doc for HexDocs.pm will become the source of truth for this Elixir
library and leverage on latest features of ExDoc.

doc: misc doc changes