add more detail around cross-zone load balancing and NLB security groups for PL setups

[dhaworth]

What are you changing in this pull request and why?

There are a couple of finer details around certain PrivateLink setups that haven't been included in the setup docs that have caused customers to get tripped up. This PR will add:

1. A note about enabling cross-zone load balancing for their NLB or target group (done through a snappet since it's the same on all 3 pages and could apply to either NLB or Target Group
2. A section to the NLB setup about Security Groups (added inline in the SG section since it only applies to NLB).

Additionally, I added some NLB info that was on the VCS page, but not Redshift or Postgres. These details are applicable to any of them.

Feel free to suggest a different format for any of these changes.

Checklist

• I have reviewed the Content style guide so my content adheres to these guidelines.

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Name	Status	Preview	Updated (UTC)
docs-getdbt-com	✅ Ready (Inspect)	Visit Preview	Oct 1, 2024 3:06pm

[matthewshaver]

Suggested change

- **Security Groups:** The Network Load Balancer (NLB) associated with the VPC Endpoint Service must either not have an associated Security Group, or the Security Group must have a rule that allows requests from the appropriate dbt Cloud **private CIDR(s)**. Note that **this is different** than the static public IPs listed on the dbt Cloud [Access, Regions, & IP addresses](https://docs.getdbt.com/docs/cloud/about-cloud/access-regions-ip-addresses) page. The correct private CIDR(s) can be provided by dbt Support upon request. If necessary, temporarily adding an allow rule of `10.0.0.0/8` should allow connectivity until the rule can be refined to the smaller dbt provided CIDR.

- **Security Groups:** The Network Load Balancer (NLB) associated with the VPC endpoint service must either not have an associated security group, or the security group must have a rule that allows requests from the appropriate dbt Cloud **private CIDR(s)**. Note that _this is different_ than the static public IPs listed on the dbt Cloud [Access, Regions, & IP addresses](https://docs.getdbt.com/docs/cloud/about-cloud/access-regions-ip-addresses) page. dbt Support can provide the correct private CIDR(s) upon request. If necessary, until you can refine the rule to the smaller CIDR provided by dbt, allow connectivity by temporarily adding an allow rule of `10.0.0.0/8`.