Fix buildAndCopyUI and update build instructions

[Gold856]

Description

The Gradle plugin we use for invoking Node and npm seems to not support the same tasks for pnpm. Define our own task for building the UI. Add install instructions for pnpm because you need it installed globally for the task to work. (minor fix to code block because it was easy to fix while updating the instructions.)

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

[samfreund]

Do we need pnpm only or pnpm@exe? I have both installed right now, so I'm not quite sure how to check.

Only pnpm is needed, the only difference is that pnpm@exe is self-contained, they have the same functionality.