Modify translation link to Crowdin/readthedocs

The https://openrocket.info/contribute.html currently links to http://openrocket.trans.free.fr/index.php?lang=en, which has been outdated for a long time. We should link to the new Crowdin translation site (https://crowdin.com/project/openrocket), or our readthedocs (https://openrocket.readthedocs.io/en/latest/dev_guide/contributing_to_translations.html).