Doc tests failing in CI

[sampullman]

Doc tests are failing in CI on the rustc beta channel: example

To Reproduce
Submit any PR that triggers the CI workflow in Github actions.

Expected behavior
Doc tests pass

Notes

This is probably caused by skeptic, which appears to be unmaintained. In order to get doc tests working, it's probably necessary to either:

1. Fix in skeptic and hope the maintainer is available to publish a release
2. Switch to [builtin documentation tests. I'm not sure what special features of skeptic are used, or how much work the transition would take.

[a1phyr]

Native doc tests would not really work here as skeptic is used to test the README and the doc quide.

[sampullman]

Temporary fix in #1274, keeping this open until a long term solution is found.

[sampullman]

It's not as clean as skeptic, but as of 1.54 it should be possible to manually include all the doctests using #[doc = include_str!("../../README.md")] (seen here).

Some more work would need to be done to ensure test compatibility and rewrite or remove doc_tests/build.rs. A solution would also have to be found for skeptic template usage.