Document Procedure to test IDE updater with local server

Add some documentation about this: https://github.com/arduino/arduino-ide/pull/865#issuecomment-1051039227

Check also Per docs: https://github.com/per1234/private-tooling-team/blob/ide-manual-test-suite/docs/manual-test-suites/arduino-ide/updates.md#use-local-update-server