Add besselj0 equation and rename bessel-first-kind link

[rreusser]

Checklist

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• Read, understood, and followed the contributing guidelines, including the relevant style guides.
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Description

What is the purpose of this pull request?

This pull request:

• Adds an equation to besselj0
• renames the bessel first kind link from bessel-j to bessel-first-kind, which will probably overlap with this change being made in another branch already, but it's not in the develop branch so that should come out in the wash.

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@stdlib-js/reviewers