Add mruby topic

[chadowo]

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

---

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

Even though it is not as popular as Lua, mruby has been used in a great deal of projects, like video games, desktop applications, IoT devices and, (most particular) in the embedded field. It would be nice if it this topic was curated so that more people can know about it.

---

Please note: we will close your PR without comment if you do not check the boxes above and provide ALL requested information.