disallow Big Tech crawlers for AI training data sets

There are plenty of opportunities for app developers to have their data indexed for AI training sets. f-droid.org should opt out so that app developers have an easier time opting out if they want to.

• https://platform.openai.com/docs/gptbot
• https://developers.google.com/search/docs/crawling-indexing/overview-google-crawlers#google-extended
• https://commoncrawl.org/faq
• https://mashable.com/article/google-bard-ai-opt-out-websites