docs/quickstart: Use higher-level headings

Use a level 1 heading as the top-level heading in the Quick Start guide.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
docs/quickstart: Tweak Introduction section

Drop the first second level heading as superfluous. In the feature
listing, group single thread and non-blocking I/O together. Fix the
name of SystemTap.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>