Support subdirectories in build mode

[SebastienBocquet]

Hi,
I noticed that when subdirectories are used in the gallery, mkdocs build creates broken links (click on the example icon leads to broken link, and showing an image in an example does not show the image (the link to the image is broken)).
I understand from the doc that the build mode is not supported or not fully.
I find this mode useful when deploying a doc on an internal server (we can copy the generated site directory on the server).
I am wondering if a full support of the build mode useful to other users?
If so, I can take some time to contribute.
What do you think?