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. Note that the
listing of supported protocols is not actually exhaustive. 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>
docs/quickstart: Tweak the FIX section

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