Linter: check remote sources

This problem comes up with Quarto itself, and I bet is very common in other sources of technical documentation.

How can we check that the downstream source of a piece of documentation is consistent with what the documentation is stating? In Quarto, this came up with a discrepancy between our SCSS documentation and our code (https://github.com/quarto-dev/quarto-web/commit/0e49089e4755c241851b2ebf01cb831bc9cf0d94, see #6209).

Then I ran Chap. 20 of this book, which talks about "IfThisThenThat", a feature of some of Google's internal tooling.

Our usecase in quarto is a little different, but we could have our (future) linter recognize syntax for checking a different remote source for consistency (I can imagine checking against a commit in a github or gitlab repo, for example). Then, quarto lint could issue a warning about inconsistent documentation. In CI, that would probably be good enough warning.