Add Rexx topic

[sparrac]

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.

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

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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

This topic (Rexx) represents an established programming language with a long history, official documentation, and an active user community. Curating it improves discoverability and helps users find repositories related to the language.

[github-actions]

Maintainer triage

Topics

• netrexx — 7 repositories — topic page
• oorexx — 19 repositories — topic page
• rexx — 70 repositories — topic page