minor: example of run for diff_antlr.sh and hack avoid pull of forked repo

[romani]

this helped me to run this script on local for https://github.com/checkstyle/checkstyle/pull/14803#issuecomment-2473856904

Not for merge!!!

[nrmancuso]

No need to run anything on local anymore, just clone this and change a few lines: https://github.com/nrmancuso/checkstyle-diff-report-generator