[DOCS] Clarify deprecation of Kibana settings

[lcawl]

Related to #29496 and #21928

Describe the bug:

Some settings (e.g. elasticsearch.url) disappear from the 6.6 documentation but are not marked as deprecated in earlier versions of the documentation. For example, see https://www.elastic.co/guide/en/kibana/6.5/settings.html (which contains elasticsearch.url) and https://www.elastic.co/guide/en/kibana/6.6/settings.html (which contains elasticsearch.hosts, not elasticsearch.url). We should add https://github.com/elastic/docs#additions-and-deprecations to give advance notice that the settings are being deprecated.

[elasticmachine]

Pinging @elastic/kibana-platform

[KOTungseth]

@lcawl can we close this one?