When :enabled-for-modes is nil we default it to '(prog-mode text-mode), but if
any of them is in :disabled-for-modes then it shouldn't be enabled by default.
This commit fixes the second part.
To avoid further issues with the publishing of
documentations I have written a test to ensure that
all documentation files can be exported to html.
To avoid issues with loading the right org version
after package installation I have also slightly
modified the test workflow to include a dedicated
package installation target done prior to executing
any tests.
