Backport bug fixes to 2.4.x

[shaobo-he-aws]

Description of changes

Backported

• Cleanup in README.md and CONTRIBUTING.md #383
• Use cedar-policy README.md as crate top-level documentation #386
• Fix PolicySet::link to avoid mutating policy set on link fail #412

Issue #, if available

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A bug fix or other functionality change requiring a patch to cedar-policy.

I confirm that this PR (choose one, and delete the other options):

• Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

Disclaimer

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[shaobo-he-aws]

We need to backport #383 first in order to #386 to work because the latter will enable doc test in the README which wouldn't be correct without the former.

[john-h-kastner-aws]

Look like one of the rust-doc lints started failing. I guess it's just because of an update to clippy at some point.

Should just need to add back-ticks where it asks for them

[shaobo-he-aws]

Look like one of the rust-doc lints started failing. I guess it's just because of an update to clippy at some point.

Should just need to add back-ticks where it asks for them

Yes. I fixed it in #670.