Make pageContext._pageId part of the official API

[brillout]

Description

Many users use the internal property pageContext._pageId for various reasons.

It should be renamed pageContext.pageId while considering changing how the page ID is determined to be a breaking change.

We should make the page ID more stable. A fairly stable page ID would be to take the Filesystem Route as ID.

I'm postponing this until the new VPS design is released. In the meantime you can safely use pageContext._pageId as I won't break it. (The new page ID will live at pageContext.pageId while the current ID will still live at pageContext._pageId.)

If there is interest, VPS could allow users to name pages, ensuring that the page name doesn't change no matter where the page file lives. (With the proposed design, when the user moves files around, then the page ID changes.)

[brillout]

Postponing after #341.