Missing Docs: pages:deploy job and stage

Problem to solve

After an issue flared up in our pipeline, and discussed in slack, we realized that there is no documentation describing the pages:deploy job and deploy stage, which are automatically generated by Pages. This needs to be added.

As noted in the chat:

Marcel Amirault  4 hours ago
To be clear, I'm not sure where the pages:deploy job and deploy stage are coming from,
but I'm hoping that properly excluding the pages job will pull out the pages:deploy job as well.

remy  12 minutes ago
pages:deploy is a “magic” job that runs when a pages job exists. I notice this isn’t documented at all!

Marcel Amirault  11 minutes ago
And yeah, I checked the documentation for this, assuming it was something
automatically done for pages, even grep'ed through the docs because it "must"
be there, but didn't find anything at all!

remy  11 minutes ago
No, I grep’ed the whole project, there’s one occurrence in the changelog,
one in a spec, and one in the actual service that create this job.