[4.6.0] Remove Kibana and Splunk from documentation

[Tostti]

Description

This PR aims to remove the Splunk and Kibana sections from the documentation, as there won't be more Kibana and Splunk apps from v4.6.0. Resolves issue #6399

Checks

Docs building

• Compiles without warnings.

Code formatting and web optimization

• Uses three spaces indentation.
• Adds or updates meta descriptions accordingly.
• Updates the redirects.js script if necessary (check this guide).

Writing style

• Uses present tense, active voice, and semi-formal registry.
• Uses short, simple sentences.
• Uses bold for user interface elements, italics for key terms or emphasis, and code font for Bash commands, file names, REST paths, and code.