Update "Ruby on Rails 5.2 Release Notes" Guide [ci skip]

- Add a description of major features in Rails 5.2.
- Add a reference to Pull Request/Commit to every entry in CHANGELOGs,
  note that some of them combined.
- Add section "Ruby on Rails Guides" with notable changes.
- Note that
  - Skipped this since encrypted secrets are already on the way out.
    ```
    *   Add `rails secrets:show` command.
        ([Pull Request](https://github.com/rails/rails/pull/29695))
    ```
  - Skipped this since it was backported all the way to 5-0-stable.
    ```
    *   Make Rails' test runner work better with minitest plugins.
        ([Pull Request](https://github.com/rails/rails/pull/29572))
    ```

Thanks to everyone who has been working on Rails 5.2! <3

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