[docs] update RDS guides

[GavinFrazar]

This is a docs only PR that updates our RDS guides.

For the RDS Aurora/instance guide:

1. condenses the RDS guide's teleport db configure commands from 2 to 1
2. eliminates the unused region variable and the unnecessary helm chart region variable (teleport will derive it from the uri)
3. changes the example database name to "rds-example" instead of a configurable variable. The database name isn't relevant to the guide and we shouldn't use a variable for because it messes up the example tsh db ls output table format.

For RDS Proxy:

1. split up the first step, which was doing a lot, into separate steps: install teleport > configure teleport > configure aws credentials.
2. removed outdated explanation about teleport db configure create - the example command doesn't output the deprecated db discovery config

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[greedy52]

Unrelated. We should fix this ordering:

We used to order by alphabet but now it's quite random. And some starts with AWS and some does not.

I think we should order by usage and what we want to promote.

[ptgott]

Suggested change

	<TabItem label="Teleport Enterprise (cloud-hosted)">
	<TabItem label="Cloud-Hosted">

[GavinFrazar]

Unrelated. We should fix this ordering:

We used to order by alphabet but now it's quite random. And some starts with AWS and some does not.

I think we should order by usage and what we want to promote.

@ptgott @greedy52
Is adding a star emoji too much? e.g.:

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🤖 Vercel preview here: https://docs-5wayqijcc-goteleport.vercel.app/docs/ver/preview