Add indicator that a PR has been included in the latest staging build

At the moment, as [documented](https://kernelci.org/docs/instances/staging/) the staging pipeline automatically goes through each relevant kerrnelci repository and tries to merge [valid](https://kernelci.org/docs/instances/staging/#github-pull-requests) pull requests into a staging.kernelci.org branch for each repository.

These branches are then used in a "staging" version of kernelci.org - staging.kernelci.org.

The process occurs every 8 hours and it works great.

However, when there are lots of PRs for the same repository there is always a chance that there may be a merge conflict. If this happens the branch/PR being merged will be ignored. When this happens there is no way for the user to know that their PR hasn't been picked up by staging in the GitHub GUI unless they look through the commit log.

The goal for this ticket is to add a visible way to determine whether a PR is part of the latest staging.kernelci.org build.

This could be done by adding a new `in-staging` label, or by adding a comment, or both, or some other method.

Start with the staging pipeline documentation at kernelci.org/docs/instances/staging/ and its GitHub pull requests section. Trace how valid pull requests are merged into each staging.kernelci.org repository branch, including the merge-conflict path. Done means users can determine in GitHub whether a pull request is included in the latest staging build.

Written by the indexing model from the issue text.