docs(build): clarify docs building instructions

Using virtual environments with pip is a generally recommended good
practice but the docs do not acknowledge it. As a result fresh installs
might fail builds due to missing $PATH entries. The Prerequisites
section is also a bit verbose which is difficult to read.

This patch adds the virtual environment mention and clarifies wording.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Iea447fb59dc471a502454650c8548192d93ba879