[GitHub] Allow shortcut for "introductory issue" and request linking to issue in PR (#84635)

The answer to many requests in issues to be assigned to users is often
"just create a pull request". That's in contradiction to the
"introductory issue" instructions posted by the GitHub bot.

This change updates the instructions, mentioning the shortcut of "just
creating a PR". Moreover, it now explains linking PRs to issues in order
to close them automatically upon merge.