website: Indicate that installing protobuf is optional

- Explicitly say that protobuf is optional.
- Elaborate more on a possible protobuf error.

JIRA: https://gem5.atlassian.net/browse/GEM5-1032

Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Change-Id: Ie0fab8e46827b0b880814a433b7b9fdc55ceb664
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/48060
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>