Improve Installation and Integration sections

[wolfgangwalther]

Both installation and integration are related - this is all about how PostgREST works together with other systems, no matter whether that's the operating system (installation) or other services (integrations).

The postgrest-docs repo had the following issues in this area still open:

Installation:

• [Feature Request] systemd scripts postgrest-docs#386
• Add instructions/configurations for using/scaling postgrest on different clouds postgrest-docs#453
• Move binary installation from tut0.rst to install.rst and add install options to tabs postgrest-docs#707
• Chocolatey doesn't add postgrest to the PATH postgrest-docs#718
• Explain how to test whether the installation was successful.
• Explain how to take the next steps on configurating to get something useful.

Integrations:

• Configuration for keycloak postgrest-docs#203
• Audience parameter required by Auth0 postgrest-docs#229
• Use Openid Connect Provider with Postgrest postgrest-docs#474
• swagger-ui postgrest-docs#475