Update index.md with C# and C++

[matthew0420]

Please confirm this pull request meets the following requirements:

• [x] I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• [x] I have no affiliation with the project I am suggesting (as a maintainer, creator, contractor, or employee).

Which change are you proposing?

• [x] 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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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• [ ] Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• [x] Content (and my changes are in index.md)

Please replace this line with an explanation of why you think these changes should be made.

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