[CI] Helm chart workflow fix

[sotojn]

This PR makes the following changes:

• Adds a GitHub App token to the build-and-publish-helm-charts ci job with the correct permissions
  • This fixes an issue where the job would fail because it couldn't push the index.yaml into the master branch
• Adds ignore path to the merge to master pipeline because Github Apps can trigger other ci pipelines
  • This specifically ignores re-triggering the pipeline in the event index.yaml is merged with master
• Makes change to build-docs ci job that will checkout the latest changes from the master branch
  • This resolves an issue where in a previous pipeline we push to the master branch and don't receive the expected changes that are necessary for the build-docs job