Make report/export/share actions per-app pages that use snippets rather than a centralized generic page

Today we have a central "report and share" page. It lives IMO too far from where it's needed. Users want to export dashboards, discover sessions or elements, visualizations, etc. So the sharing information should be available in each of these docs, tailored with the options actually available for each.

This issue is to explore the feasibility of this idea, leveraging snippets over centralized information that's worded too vaguely to be generic enough.

the centralized page comes from the fact that "reporting & sharing" is seen as a kibana module technically, and its configuration on some environments is done as such, but it does not mean users think of sharing as a generic piece. What's being shared/exported should be the focus.

We've recently had issues/comments around dashboard sharing/exports where we've had to provide more details in dashboards docs themselves, so this is what sparked this idea as something we can generalize to make consistent.