Create post upgrade task function and command to regenerate readme with helm-doc

Add a function to post upgrade tasks functions.sh to regenerate the README with helm-doc and add the command to allowedPostUpgradeCommands. This will then be added to every package renovate.json in this issue https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/1110 to automatically regenerate for renovate update MRs