Docs for multiple buckets with varying properties, Digital Ocean signed URLs with CDN domain

[awhileback]

from guide posted in #944, also referenced in #395

I double checked and cleaned up the example template filter, but I don't use DO Spaces and would rather not pay 5 bucks to test this if possible, perhaps @lifenautjoe can verify that this all works on his DO account.

[paulkuhle]

I followed the steps in the updated docs and can verify that they work perfectly.

One minor thing: I had to also add the storage bucket name to the replace function, given that django-storages generates the link in this format (https://fra1.digitaloceanspaces.com/example-cdn instead of https://example-cdn.fra1.digitaloceanspaces.com).

This is a quick example of how this would look like in a graphene resolver (I imagine the same principle applies to template filters, DRF endpoints, etc. - maybe it would be a good idea to make this section more generic in the docs?):

def resolve_file(parent, info):
    url = parent.file.url
    if settings.AWS_S3_ENDPOINT_URL in url:
        cdn_domain = 'https://cdn.example.com'
        new_url = url.replace(
            f"{settings.AWS_S3_ENDPOINT_URL}/{settings.AWS_STORAGE_BUCKET_NAME}", cdn_domain)
        return new_url
    return url

It would be great if we could get this merged soon given that it's really valuable information. 👍