Mend the test prerequisites, document local CCTV packs without the dead demo feed, and credit the author

changelog.d/210-contributor-docs-refresh.md

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+- Contributor docs: CONTRIBUTING lists what each of the three baseline checks
+  needs to run, DATA_SOURCES documents loading a local CCTV source pack, and
+  KNOWN-ISSUES points the floor-placement note at its verification script
+  instead of a missing report (Gaurav Patidar, @gaurav7902, #210).