Fix long first paragraphs in docs and deny clippy warning

Description of changes

Applies a new clippy suggestion for nicer docs and adds this new lint to our list deny lints.

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

[ ] A breaking change requiring a major version bump to cedar-policy (e.g., changes to the signature of an existing API).
[ ] A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).
[ ] A bug fix or other functionality change requiring a patch to cedar-policy.
[x] A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)
[ ] A change (breaking or otherwise) that only impacts unreleased or experimental code.

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

[ ] Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).
[x] 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):

[x] Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.
[ ] Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-spec, and how you have tested that your updates are correct.)
[ ] Requires updates, but I do not plan to make them in the near future. (Make sure that your changes are hidden behind a feature flag to mark them as experimental.)
[ ] I'm not sure how my change impacts cedar-spec. (Post your PR anyways, and we'll discuss in the comments.)

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

[x] Does not require updates because my change does not impact the Cedar language specification.
[ ] Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-docs. PRs should be targeted at a staging-X.Y branch, not main.)
[ ] I'm not sure how my change impacts the documentation. (Post your PR anyways, and we'll discuss in the comments.)