minor: DOC: add group creation instructions to LZ and Guardrails docs - for users new to GCP

Some users are having issues creating the 3 group accounts (embedded Guardrails group users in the LZ solution)
The yaml is hardcoded to accept only group: not user: prefixes when adding IAM roles around the groups.

You may be referring to the 3 recommended guardrails groups (part of the lz) - you can create all 3 in admin

https://admin.google.com/ac/groups

or console and add your org admin/super-admin users to all 3 - then you will be ok with the "group:" prefix instead of the "user:" prefix - or adjust all the role prefixes to be user:
see section 4 of https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/blob/dev/solutions/landing-zone/README.md#setup

Discussion
```
groups can be created either in console or admin (current doc on this happens to be in admin - migration of instructions in the queue for gcp)
optionally, Or just edit your setters.yaml - like we discussed before to one of the default groups (postmaster) - until you need to see generated emails (log sink errors etc....)

michael@cloudshell:~/wse_github/GoogleCloudPlatform$ cat landing-zone/setters.yaml
# Groups
# Permissions will be assigned to the specified group email
audit-viewer: postmaster@gcp.zone
log-writer: postmaster@gcp.zone
log-reader: postmaster@gcp.zone
organization-viewer: postmaster@gcp.zone

Right now the yaml code is hardcoded to group: I would just put in the fake groups for now - creating a group is
- go in admin or console, create the group, add one of your SA users

Group Name
Group Email
Description

abuse
abuse@gcp.zone
audit-viewer
audit-viewer@gcp.zone
postmaster
postmaster@gcp.zone

https://console.cloud.google.com/iam-admin/groups?organizationId=925207728429&orgonly=true&supportedpurview=organizationId,folder,project
```