Enforce no local editing of files that are maintained elsewhere

[mdlinville]

Problem description

We want reference docs to be maintained in their code repos, along with
the code they are referencing. We want to periodically bring them up to date
in this repo using automated processes.

Two things need to happen:

• Create a manifest of directories and files that should not be the target of pull requests in this repo: Done in PR Add manifest of files maintained elsewhere #209.
• Enforce that manifest using CI, on each pull request

The automated refreshing of the reference docs in this repo is out of scope for this issue, but
a second issue can be filed as a follow-up to this one.

/cc @thaJeztah @stevvooe @mikedougherty @johndmulhausen

[mdlinville]

We did this by removing those files from our repo and getting them at build time.

[docker-robott]

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