Deprecate -doctest attribute

[williamthome]

This PR deprecates the -doctest attribute to simplify the API, reduce lib maintenance, and give more control over the tests.

This is no longer valid:

-ifdef(TEST).
% The doctest header sets `doctest_transform` as a parse_transform:
-include_lib("doctest/include/doctest.hrl").
-doctest #{
    % Options
}.
-endif.

Instead, do this:

-ifdef(TEST).
-include_lib("eunit/include/eunit.hrl").

% The EUnit header will automatically export this function because it ends with "_test"
doctest_test() ->
    doctest:module(?MODULE, #{
        % Options
    }).
-endif.