Rework Pages documentation to use `pages: true` as default instead of `pages` job name

The GitLab Pages documentation relies heavily on examples where a pages deployment is triggered by defining a job with the name pages. The text also refers to that in a couple of places.

With !166731 (merged) this is no longer necessary.

I believe we should rework the docs to that:

• all examples use pages: true instead of pages as a job name
• naming the job pages without specifying pages: true should be communicated as deprecated
• and find other occurrences hinting to the old ways and fix them