text: Title: Remove docs for other projects from the Scrapy docs
  Body: We have some auto* statements for things from other projects, like itemadapter in https://github.com/scrapy/scrapy/blob/92f86fab062d179fd68590f1be6a24ca13ec33b6/docs/topics/items.rst?plain=1#L404 and parsel in https://github.com/scrapy/scrapy/blob/92f86fab062d179fd68590f1be6a24ca13ec33b6/docs/topics/selectors.rst#L1037.
  
  We should find these and remove them, instead linking to the other projects docs and maybe adding some explanations locally.