Add docs with networking scenarios

While working on #114 I have experimented with different networking arrangements and have been reminded that it is easy to create a configuration that does not work! It could be helpful for users to see a few example configurations while they consider their own deployment.

Here are a few that do work:

## Scenario 1: Higher security, higher cost
* RDS Instance in a `PRIVATE_ISOLATED` subnet
* [optional] `pgbouncer` instance in a `PRIVATE_WITH_EGRESS` subnet
* `pgstac_secret` host value is the private IP address for pgbouncer or the RDS hostname
* Lambdas added to the `vpc` in a `PRIVATE_WITH_EGRESS` subnet (adds a NAT Gateway which costs $)
* granted connect access to either the RDS instance or the pgbouncer instance
* **important**: add an S3 Endpoint to the `vpc` to make it possible to read data from S3 buckets in the same region as the VPC for free even within the vpc instead of paying the NAT Gateway tax

## Scenario 2: Lower security, lower cost
* RDS Instance in a `PUBLIC` subnet
* [optional]`pgbouncer` instance in a `PUBLIC` subnet with a public IP address assigned
* `pgstac_secret` host value is the public IP address for the `pgbouncer` instance or the RDS hostname
* **important**: RDS Instance (or pgbouncer instance) allow all traffic (any Ipv4) on port 5432
* Lambdas in `PUBLIC` subnet
* Makes it possible to connect to the database from any client with the credentials
* convenient for loading STAC items without the ingestor infrastructure