main: Create publishing PRs in new site 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.

Test PRs created by these workflows:

• https://github.com/OAI/spec.openapis.org/pulls

Counterpart to

• main: Create publishing PRs in new site repo OpenAPI-Specification#5046
• main: Create publishing PRs in new site repo Overlay-Specification#156

Part of

• Should we-organize our GitHub pages to a separate repo (by way of a directory on main)? OpenAPI-Specification#3717

[frankkilcommins]

Looks good. Thanks @ralfhandl