Publish doc on rtd

[jburel]

Build the doc on readthedocs

This is currently available at https://readthedocs.org/projects/jburel-ome-model/ The link in the README will need to be updated when we have the final url

https://docs.readthedocs.io/en/stable/versions.html#states for version I have also not activated the webhooks.

cc @sbesson

[jburel]

for the version: this is the current one in the file. When a tag is pushed the version X.X.X will be shown (as it is now). This can easily be removed. I have not looked at that aspect in this PR, only the required changes to work with rtd. I think we should consider hiding latest i.e. the dev work. The CI infra will certainly need some TLC, I should have mentioned that in the description.

Having the doc independently will make sense also the layout used here docs/sphinx is not used in other repositories

[sbesson]

Discussed today. The current consensus seems to lean toward extracting this documentation and maintaining it separately. @jburel will come back to this when possible. In the meantime, excluding it to reduce the build failures.

[jburel]

The release of the documentation is still done via https://ci.openmicroscopy.org/job/OME-MODEL-release We are shutting down ci.o.org. We need to get back to this work

I will have a look and see if I can use the same strategy than the one used of BF i.e. build using ant and sphinx

[jburel]

I have adjusted the build. Reading the previous comment, extracting will probably take a bit more time but can do that