feat: add `https_enforced` GH Pages setting

[matijs]

Resolves #2276

---

Before the change?

• n/a

After the change?

• it's possible to set https_enforced

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

---

[matijs]

It looks like this is a little more involved and depends on cname being set. For Pages sites without a custom domain, https is always enforced.

[matijs]

It looks like this is a little more involved and depends on cname being set. For Pages sites without a custom domain, https is always enforced.

Updated the PR accordingly.