Add isPage() fn to useLinkGen() hook

[emilydrakesmith]

Describe your changes

• Reorganize the return statement in the useLinkGen() hook
• Add a new isPage() method to the logic housed and returned by the useLinkGen() hook

Link the related issue

Closes #3611

Checklist before requesting a review

• Is this PR ready for merge? (Please convert to a draft PR otherwise)
• I have performed a self-review of my code.
• Did I request feedback from a team member prior to the merge?
• Does my code following the style guide at docs/CODING-STYLE.md?

Instructions for Reviewers

Functionalities or workflows that should specifically be tested.
I refactored App.tsx to use the new logic to determine on what pages the left sidebar should render. Poke around the app, look for pages where the sidebar appears but shouldn't or fails to appear when it should.

Environmental conditions that may result in expected but differential behavior.
None!

If relevant, list additional work to complete pre-merge (delete logging, code abstraction, etc)

This goal of this PR is to make new centralized logic available for ongoing development and instantiate it in App.tsx for proof-of-concept. The app will still need a wider refactor to replace all instances of old logic with the new centralized process.

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[emilydrakesmith]

Sample old vs new logic: