Add PR labeler

[Gold856]

Description

It's nice to be able to filter PRs by what components they've modified. The 5 labels that have been selected are frontend, backend, documentation, photonlib, and website, since those are the primary components in the monorepo.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2024.3.1
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added

[mcm001]

test it in prod