improve special remotes list

improve special remotes list

In several cases, there's an old, maybe unmaintained special remote,
and rclone also supports the service. I don't know which is better,
so I added tips pages for these, listing both possibilies, so users can
fill in the details.

Added some things listed in special_remotes/rclone that were not
in the cloud provider list, so users will see it supports those.
But, omitted S3 and webdav, since it's probably better for users to use
what's built into git-annex than going via rclone.

The rclone list is out of date apparently, as rclone.org has more,
so I added a note at the bottom for users to check.

This commit was sponsored by Ethan Aubin.