Documents pagination

Motivation: Add pagination support

These changes add pagination support to documents/ endpoint.

Users of this app (django_minutario_client) that have PAGE_SIZE into settings.py like:

REST_FRAMEWORK = {
    PAGE_SIZE = 100,
    ...
}

will get paginated responses. If PAGE_SIZE is not set, it will behave like before, therefore not impacting new users, even if this app is updated.