Update and restructure PostgreSQL integration docs.

[alistairmatthews]

The existing PostgreSQL integration document is comprehensive but difficult for beginners. This PR is intended to keep the comprehensive information but make it easier for those who want to get going quickly.

This PR:

• Splits the large integration doc into two: Hosting and Client.
• Adds a getting started short tutorial.
• Adds a getting started tutorial for both hosting and client integration.
• Adds a Properties of the PostgreSQL resources section to the client reference docs. This describes what configuration values are injected by the WithReference() method. I've put this in the Client doc, where you might use them, but added a Note and a link in the Hosting doc.

Addresses: #30, #9

[IEvangelist]

This is a good start, let's make the following updates:

• The "get started" article per integration should be simplified so that it's just the brief intro, hosting integration package install instructions, the primary API to add the resource, link to advanced configs and scenarios (hosting reference it's called now), details of what config is injected, and then a client section that has a language pivot. C#, Python, and JS...

No need for the client standalone article.

[alistairmatthews]

This is a good start, let's make the following updates:

• The "get started" article per integration should be simplified so that it's just the brief intro, hosting integration package install instructions, the primary API to add the resource, link to advanced configs and scenarios (hosting reference it's called now), details of what config is injected, and then a client section that has a language pivot. C#, Python, and JS...

No need for the client standalone article.

@IEvangelist - thanks very much. I've implemented all that in the Get started article. The Python pivot may be a little sparse. I haven't deleted the Client integration document yet - could you confirm that you don't mind losing sections like Configuration, Client integration health checks and so on?