feat: Add the backend-br topic. - #5428

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• This change is not self-promotion.

I left this box unchecked because I help maintain the Back-End Brasil organization. It is a community project with outside contributors, and the topic is already used by repositories outside the organization.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

Curating a new topic or collection

• I've formatted my changes as a new folder directory, named for the topic or collection as it appears in the URL on GitHub (e.g. https://github.com/topics/[NAME] or https://github.com/collections/[NAME])
• My folder contains a *.png image (if applicable) and index.md
• All required fields in my index.md conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

The backend-br topic is used by 14 public repositories from three owners: the Back-End Brasil organization, buildrun-tech, and me. Back-End Brasil is a community whose job board, backend-br/vagas, has more than 8,000 stars. Curating the topic gives its page a name, a description, and a link to the community site.