Add cfd (Computational Fluid Dynamics) topic

[oxygen-dioxide]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• I am not the sole author or employee of a company who created either the topic I am modifying/adding or the collection entry I am modifying/adding.

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

Computational Fluid Dynamics (CFD) is an important application of computer technology in engineering. There are many repos about CFD on GitHub.

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

[oxygen-dioxide]

Also, when will our changes show up on GitHub's topic pages after they're merged? My PR #4611 (yugioh) was merged one month ago but still doesn't show up on https://github.com/topics/yugioh