[DOCS] Adds settings section to breaking changes

[lcawl]

Summary

This PR categorizes the entry in the 8.0 Breaking Changes (https://www.elastic.co/guide/en/kibana/master/breaking-changes-8.0.html) into a "Settings changes" section, so that it matches the layout established in 7.0.

This change also ensure that the info is displayed properly in the Installation and Upgrade Guide. Currently, it's appearing as a peer to the Kibana section in https://www.elastic.co/guide/en/elastic-stack/master/elastic-stack-breaking-changes.html

[skip ci]

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

- [ ] This was checked for cross-browser compatibility, including a check against IE11
- [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support

• Documentation was added for features that require explanation or tutorials
  - [ ] Unit or functional tests were updated or added to match the most common scenarios
  - [ ] This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-docs

[schersh]

LGTM - thanks