Improve the user flow for editing the (autogenerated) docs

The Nushell website has links at the bottom of the page to "Edit this page on GitHub". Clicking the link takes the user through the process of updating that page using the GitHub web interface to create a pull request. That user flow bypasses the CONTRIBUTING.MD and it's not until after the PR is submitted the user is notified about not updating autogenerated docs.

Some suggestions to improve this for first time contributors.

1. Remove the "Edit this page on GitHub" links on pages that should not be edited.
2. Change or create the GitHub issue template specifying that certain pages, trees roots or paths should not be modified.
3. Add a note about contributing to the README.md. As it is, the user is expected to know about the CONTRIBUTING.md document.

Leggi prima README.md e CONTRIBUTING.md, quindi analizza come il sito web aggiunge i suoi link «Edit this page on GitHub» e come sono configurati i template delle issue di GitHub. Il lavoro è completo quando le pagine che non dovrebbero essere modificate non mostrano più questi link, il template della issue identifica i percorsi generati automaticamente e README.md indirizza i contributori alle indicazioni per contribuire.