Improve CI around docbuilds

[aaronjeline]

Describe the improvement you'd like to request

4.2.0 launch accidentally had a build error in our docs that CI didn't catch. It would be nice to catch this.

Describe alternatives you've considered

No response

Additional context

No response

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this internal improvement
• ⚠️ This feature might incur a breaking change

[john-h-kastner-aws]

Problematic line was here (still a problem on main at time of comment):

cedar/cedar-policy-validator/src/entity_manifest.rs

Line 71 in 1729f98

#[doc = include_str!("../../cedar-policy/experimental_warning.md")]

To detect this we would need to build the validator crate in an environment without the cedar-policy directory. I don't believe it would be enough to just cd into the validator crate and build docs from there.