docs: update README and overview per comments

- Keep "Feedback and Support" section consistent in README.md and
overview.rst.
- Remove support@arm.com, as customer support is not available yet by
Customer Support team.
- Replace 'Fast Model' with more formal 'Fixed Virtual Platforms', and
update the architecture diagram accordingly.
- Mention software traffic generator support in the future.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Willen Yang <Willen.Yang@arm.com>

Change-Id: I6c8028343c37bed292756713f460d19886c11b30