Add warning for incorrect usage of Nushell activation script

[esafak]

Unlike the other shells, the nushell activation script is not sourced. Users coming from other shells, like myself, may find the correct usage (overlay use) unexpected and waste time trying to solve the issue, so we will present them with a warning if they source it.

Many thanks to @Bahex for suggesting the solution in nushell/nushell#16182

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation