🚀 Feature: Write instructions for installing and configuring plugins

### 🔖 Feature description

### Write instructions for installing and configuring plugins

Write step-by-step instructions for installing and configuring a plugin. Include how to test the plugin if there's a generic way to do that. Organize under *Admin Guide* in the doc TOC. This is just one more set of procedures for an Admin, but it's so central to Backstage that it should have its own section.

Audience: Admin

Type: Task

The existing documentation might contain helpful source material that you can pull into this doc change. See this catalog of the existing (at the time of the CNCF tech doc analysis) Backstage technical documentation pages:
https://github.com/cncf/techdocs/blob/main/assessments/0008-backstage/backstage-doc-survey.csv

### 🎤 Context

This issue is a recommended change resulting from an analysis of the Backstage documentation commissioned by CNCF. The analysis and supporting documents are here:
https://github.com/cncf/techdocs/tree/main/assessments/0008-backstage

### ✌️ Possible Implementation

The goal is to continue setup of a production Backstage server by installing existing plugins needed by the organization.

Related material in the current doc:
- https://backstage.io/docs/getting-started/configure-app-with-plugins
- https://backstage.io/docs/integrations/
- https://backstage.io/docs/plugins/existing-plugins
- https://backstage.io/docs/conf/defining

Suggested changes:
- Consolidate the above information (and more) in step-by-step instructions for installing and configuring plugins.
- Include the instructions in the Admin Guide.
- Include this section in the "What's next?" section of the [production server installation instructions](https://github.com/backstage/backstage/issues/21916).
- The instructions can be applicable to stand-alone servers as well, but should be primarily aimed at production servers.

### 👀 Have you spent some time to check if this feature request has been raised before?

- [X] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [X] I have read the [Code of Conduct](https://github.com/backstage/backstage/blob/master/CODE_OF_CONDUCT.md)

### Are you willing to submit PR?

None

Review the existing plugin documentation at /docs/getting-started/configure-app-with-plugins, /docs/integrations/, /docs/plugins/existing-plugins, and /docs/conf/defining, along with the CNCF survey. Consolidate the relevant material into step-by-step Admin Guide instructions for installing, configuring, and generically testing plugins, then add it to the documentation TOC and link it from issue #21916’s production-server “What’s next?” section.

Written by the indexing model from the issue text.