Drop re_path from installation guide

[jaap3]

Description of the Change

Removed the bit about using re_path to add urls. It's just confusing, as a user I don't know why I should pick this option over the first one.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[jaap3]

Just a note, including the urls like this:

from oauth2_provider import urls as oauth2_urls

urlpatterns = [
    path("o/", include(oauth2_urls))
]

works just as well, and removes the need to set a namespace (app_name in the urls module does the job).

[n2ygk]

Please make the change you mentioned in your comment since keeping things simple is best:

from oauth2_provider import urls as oauth2_urls

urlpatterns = [
    path("o/", include(oauth2_urls))
]

[n2ygk]

See #1436 for more