Create article on dev channel testing flow for early bug detection

Please document a sample flow to test AMP pages at scale with dev channel on, in order to early detect bugs in components.

Due to the evergreen nature of AMP components, in the unlikely event of a bug being introduced in the released code, some live pages could break. Guidance on how to setup a testing framework that surfaces these bugs in test environments would be extremely helpful.

Currently, this testing can be done manually by visiting [the experiments page](https://cdn.ampproject.org/experiments.html) and turning on the dev-channel flag, and programmatically by setting up the cookie AMP_CANARY=1 for domain cdn.ampproject.org

Thanks,

Start with the experiments page and the documented AMP_CANARY=1 cookie behavior described in the issue. Document a sample flow for testing AMP pages at scale with dev channel enabled, including how the setup surfaces component bugs in test environments.

Written by the indexing model from the issue text.