Add guidelines with a set of basic expectations for becoming a maintainer of Pages

Summary

Add guidelines with a set of basic expectations for becoming a maintainer of Pages.

Some ideas on areas to be covered:

- namespace/project resolution through the GitLab API (where `Domain` object comes from)
- ZIP serving and the virtual file system
- authentication

Smaller features (I wouldn't require experience in all of them):
- redirects
- artifacts proxying
- handling of TLS certificates and working with LetsEncrypt
- rate-limiting
- metrics and monitoring

More context: https://gitlab.com/gitlab-org/plan-stage/plan-engineering/-/issues/1031#note_1691456332

Reference: https://gitlab.com/gitlab-org/container-registry/-/blob/master/docs/becoming-a-maintainer.md?ref_type=heads#expectations