Remove the pull_request trigger for the documentation workflow

[inickles]

Issue

The documentation workflow is triggered to run on both all pull requests and pushes to master. The former cannot (and should not) succeed on PRs from the public, even if the workflow is manually triggered by some with write permissions (Oxide). This has caused checks by public PRs to fail, such as #1488 with https://github.com/oxidecomputer/hubris/actions/runs/5838750982/job/15876474521?pr=1488.

Additionally, we probably don't want to trigger the workflow on pull requests, because it'd mean the live GH pages are updated before a PR is accepted. For example: https://github.com/oxidecomputer/hubris/pull/1489/files can be seen on https://oxidecomputer.github.io/hubris/reference/.

Fix

This PR just removes the trigger on pull_reqeusts while keeping the trigger on changes to the master branch, so the GH pages at https://oxidecomputer.github.io/hubris/reference/ should only reflect what's on master.

It might make sense to refine the trigger to run on changes to the doc directory, but this will at least resolve the issues.

Notes

The JamesIves/github-pages-deploy-action action uses the builtin GITHUB_TOKEN by default, which cannot have write permissions to the repo (https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token).

[steveklabnik]

Agreed! At some point, I standardized our various workflows, and I bet that I introduced this as a bug when doing so. Thank you!