API: content to add, or to determine where it should be placed.

Working on create this issue:

While we are accepting PRs, please wait until #2180 is merged. These changes should be made in https://github.com/expressjs/expressjs.com/tree/redesign. We’re not interested in updating the current documentation since that’s just manual work, and I don’t have time to review that.

We should move `app.settings` to `app.set` in all versions and show the table there.

- https://deploy-preview-2180--expressjscom-preview.netlify.app/en/3x/api/application/app-settings
- https://deploy-preview-2180--expressjscom-preview.netlify.app/en/4x/api/application/app-settings
- https://deploy-preview-2180--expressjscom-preview.netlify.app/en/5x/api/application/app-settings

We should unify them under the same route, `app.get` and `app.get.method`.

https://deploy-preview-2180--expressjscom-preview.netlify.app/en/5x/api/application/app-get
https://deploy-preview-2180--expressjscom-preview.netlify.app/en/5x/api/application/app-get-method

4x:
- app.router: https://github.com/expressjs/express/blob/12fae14531a78f19a2caaa5d4f58d9b01eaf3194/lib/application.js#L261 (4.22.1)