ci: make signed commits when publishing docs

[dsanders11]

• [x] I have read the contribution documentation for this project.
• [x] I agree to follow the code of conduct that this project follows, as appropriate.

Summarize your changes:

Switches the docs publishing workflow to using malept/github-action-gh-pages to align across the org (with electron/get and electron/packager), and uses dsanders11/github-app-commit-action to sign the commit.