main: Create PRs in new publishing repo

[ralfhandl]

Create publishing PRs in new site repo OAI/spec.openapis.org instead of this repo:

• Use app token for authentication
• spec name in head branch name to avoid collisions between specs
• base branch is main
• Align reviewers with TSC and OpenAPI-maintainers teams
• Consistent YAML formatting

Please check "Hide whitespace" when reviewing to minimize diffs.

Counterpart to

• main: Create publishing PRs in new site repo OpenAPI-Specification#5046