[Request] Update references to apps and pages re: solution views and classic nav updates in Kibana

[florent-leborgne]

Follow-up to https://github.com/elastic/platform-docs-team/issues/486

With the stateful navigation updates, we need to check that the way we refer to apps and pages in our UI-related guides remains true independently of which navigation users got by default (classic or solution view).

The wording that was discussed and agreed is in the issue linked at the top of the description.

This specific issue is for Observability guides.

The new navigation will arrive with 8.16, so ideally updates should be done by then.

Note: There's a related issue to update the general information about search and navigation principles in Kibana https://github.com/elastic/platform-docs-team/issues/530