DOC: Grammar and typo fixes in first_steps.dox. ...

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

doc: rework user manual ...

Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>