Update user-guide.rst

Following changes have been added -
1. Add Cmake prerequisite section.
2. Update Syncing section in user-guide.rst file.
3. Update user-guide.rst for 2022 R1 release.
4. Migrate support to Ubuntu 20.04 LTS host.

Signed-off-by: sahil <sahil@arm.com>
Signed-off-by: Himanshu Sharma <Himanshu.Sharma@arm.com>
Change-Id: I3df9ba3a7c59e4cd73cd773fffb78f5b4d2e4eb4