Add tool to scan for references of deleted or renamed READMEs

[silas-hw]

• Added .github/tooling/defunct-readme.sh, which compares the current branch against master to see if any READMEs have been deleted, and if so scans for references to that README in other files. If any are found, the script exits with an error
• CI hasn't been modified to use it yet
• Currently the script exits on the first error, but it shouldn't be hard to update it to list all errors before exiting
• Partially addresses Add checks for defunct and orphaned READMEs to CI #3037

[JacquesCarette]

I have not done shell scripting in far, far too long, so my 'approve' should be considered fairly weak. I see no immediate problem here, BUT... (Of course, I do think that the idea is good and helpful.)

[gallais]

I'm not a huge fan of the fact that master is hard coded in the script.
E.g. when we (rarely) are merging into experimental, master's status is irrelevant.