Consider allowing users to share a story starting from the page they're on

With the story branching feature, there is a way for users to open a story at a given page, that is specified as a URL parameter.

As of today, users can only share the story without this parameter, and have no way to generate a link "share this story and start from page N".

We introduced a checkbox "Share starting from this page" button in the sharing UX that would generate a link leading to the story and specific page the user is currently on.
However this would only work when sharing from desktop, as both iOS and Android trigger the native OS sharing where we can't control the UI or shared URL, which would be inconsistent.

We're removing this checkbox until we discuss further whether we want this, and what is the expected UX (https://github.com/ampproject/amphtml/pull/24980)

cc @jasti @hongwei1990 @newmuis