Make settings page more descriptive

We should have an introduction to what the plugin does and which plugins it works with. If either Jetpack CSS or Custom Javascript Editor aren't installed / activated, it should offer clear instructions on how to do so (or a one-click button).

Start by locating the plugin's settings-page entry point and existing handling for Jetpack CSS and Custom Javascript Editor. Verify the page explains the plugin and its integrations, and that missing or inactive dependencies show clear setup instructions or a one-click action.

Written by the indexing model from the issue text.