docs(external): add docs for haversine VRL function

[esensar]

Summary

Adds docs for the new haversine VRL function.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Related: feat(stdlib): add haversine function vrl#1442

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[rtrieu]

Thank you for your contribution! I left you some feedback and suggestions for your review, mainly for clarity and consistency. Please re-request a review when ready.

[esensar]

There doesn't seem to be a way to generate an array from a disjunction, but there is a way to generate disjunction from an array, so I went with that route, to reduce duplication.