GHA update

[jburel]

The aim of this PR is to be the doc up-to-date when changes occur in upstream repositories i.e.

• https://github.com/ome/openmicroscopy
• https://github.com/ome/omero-install
• https://github.com/ome/omeroweb-install

This combines with #2195 should simplify the release of the documentation

How does it work:

• Every day, the update action will be triggered e.g. https://github.com/jburel/ome-documentation/actions/runs/1521384260
• A Pull Request will be created if changes occur in the upstream repositories.

This workflow will be active when this PR is merged

cc @sbesson @joshmoore

The change in autogen file will be removed when/if we switch to rtd.

Edit: The workflow is now a bit more complicated due to use of omero db script

[joshmoore]

Mostly looks like a few if-catches for edge cases for this new build environment. Other than one minor comment about the use of "Update" 👍

[jburel]

Several PRs have been merged in omero-install and omeroweb-install.
Merging this PR so we can test the workflow and check output tomorrow