doc: west: release notes for v1.2

The main new features are a 'west grep' command and a new
'description:' field in the manifest file's 'projects:' elements.

Rather than try to keep the version number in index.rst up to
date, which it frequently is not, just delete the paragraph.

Signed-off-by: Martí Bolívar <mbolivar@amperecomputing.com>