Create documentation for "Configuring the Backstage Plugin" guide

[yada]

Describe the bug

Dear Community,
We are looking for a contribution (tech writer) to create and clarify the "Configuring the Backstage Plugin" section of the documentation. The goal is to ensure consistency, clarity, and completeness across our docs.

See: https://microcks.io/documentation/guides/integration/backstage-plugin/

Contributions and suggestions are welcome!

Attach any resources that can help us understand the issue.

For guidance and recommendations on maintaining a uniform style, please refer to the discussion in this related issue and comment: #415 (comment).

[yada]

Some content that can help the doc creation:
https://microcks.io/blog/backstage-integration-launch/
https://github.com/microcks/microcks-backstage-provider

[lbroudoux]

Can be closed now that #448 is merged.