[release calendar] Make release calendar accessible from amp.dev

Today, we have the following release-related pages on amp.dev:

- [Main schedule](https://amp.dev/documentation/guides-and-tutorials/learn/spec/release-schedule), linked from "Learn" -> "AMP HTML Specification" -> "AMP Release Schedule"
- [Interactive calendar](https://amp.dev/documentation/guides-and-tutorials/contribute/vendor-contributions/release-schedule), linked from "Contribute" -> "Third party contributions" -> "Interactive Release Calendar"
- [Screen-reader version](https://amp.dev/documentation/guides-and-tutorials/contribute/vendor-contributions/release-schedule-accessible) of interactive calendar, linked from above page

I think there is value in making it possible to access https://amp-release-calendar.appspot.com from amp.dev as well, given that we still see questions from external contributors asking about when their PRs were / will be released.

IIRC, we were thinking of making the calendar a part of the as-yet unreleased AMP status page, but I don't think that should preclude linking / embedding the page from within amp.dev. I'm filing this issue for discussion / comment 😃