Update UI standard page: Button

Problem/Motivation

The UI standard page for Buttons https://www.drupal.org/docs/develop/user-interface-standards/buttons → is still a draft.

Proposed resolution

Rewrite the page and add missing screenshots.
Check whether core modules follow this standard.

Remaining tasks

1. How-Why-Examples. Change the order of the text to first say how button should be used, then why, and then follow it up by a good example.
2. Add screenshots for missing button types.
3. Add a one sentences summary that's useful on https://www.drupal.org/docs/develop/user-interface-standards →
4. Add the correct link for the Drupal 8 API.
5. Check several core admin pages whether they use the buttons this way. Add in a comment which pages you checked.
6. If you find a admin page that does not follow this standard: make an issue to fix this.