Improve the user flow for editing the (autogenerated) docs

[NiceGuyIT]

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.

[NiceGuyIT]

The nushell repo has a good example for the template:

# Description
<!--
Thank you for improving Nushell. Please, check our [contributing guide](../CONTRIBUTING.md) and talk to the core team before making major changes.

Description of your pull request goes here. **Provide examples and/or screenshots** if your changes affect the user experience.
-->

(Of course you probably already knew that.)