[media] doc-rst: media: reordered top sectioning

Within the old section hierarchy, all doc parts has been placed under
the introduction, e.g:

* Linux Media Infrastructure API
    + Introduction
        - Video for Linux API
        - Digital TV API
        - ...

With separating the introduction sibling to the other parts
we get a more common section hierarchy:

* Linux Media Infrastructure API
    + Introduction
    + Video for Linux API
    + Digital TV API
    + ...

BTW: compacting the intro text.

This patch is on top of media_tree/docs-next

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>