chore: Add Alex to lint documentation (#26354)
This Pull Request adds [Alex](https://alexjs.com/) to our documentation. It catches insensitive, inconsiderate writing.
The original PR (https://github.com/vercel/next.js/pull/25821) is too large so I have decided to break it down into smaller PRs. This PR is the first part. Then I will continue to add the rest of the documentation in smaller PRs.
## More Information on Alex:
https://alexjs.com/
https://github.com/get-alex/alex
## Documentation / Examples
- [x] Make sure the linting passes
