Improve `pages:` job options: Move publish to pages.publish
## Problem
When [enabling multiple pages jobs](https://gitlab.com/gitlab-org/gitlab/-/issues/427755) a new `pages:` job option was created, but the MVC version of this option is quite verbose and don't contains all pages-related ci options.
## Proposal
* [x] Move [`publish`](https://docs.gitlab.com/ee/ci/yaml/#pagespublish) to `pages.publish`
* [ ] Use any value defined in `pages.publish` as an artifact automatically: Follow up issue https://gitlab.com/gitlab-org/gitlab/-/issues/519495 created
* [x] Update documentation to communicate the [`publish`](https://docs.gitlab.com/ee/ci/yaml/#pagespublish) as deprecated in favour of `pages.publish`
### Old:
```yaml
my-pages-job:
script:
- echo 'something'
artifacts:
paths:
- some-folder
pages:
path_prefix: '/stg'
publish: '/some-folder'
```
### New:
```yaml
my-pages-job:
script:
- echo 'something'
pages:
path_prefix: '/stg'
publish: '/some-folder'
```
Related to:
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/133624#note_1599881481
- https://gitlab.com/groups/gitlab-org/-/epics/10914+
- https://gitlab.com/gitlab-org/gitlab/-/issues/398145
issue
