Add documentation for preview hpb with Imaginary

[CarlSchwan]

Signed-off-by: Carl Schwan [email protected]

[solracsf]

Provide a working docker command here would be a good thing:
https://github.com/h2non/imaginary#docker (like, is the -enable-url-source param necessary?)

[sebalis]

Will there be some guidance on how to extend a more or less standard installation using Docker Compose? I am guessing that a new image will have to be added and some configuration in the Nextcloud container changed and would appreciate some help with the details.

[CarlSchwan]

Here is an example with docker compose: juliusknorr/nextcloud-docker-dev#59 I'll try to add this to the doc too