Add three Rust playground resources

[Tejas-Santosh-Nalawade]

• Add Codapi (https://codapi.org/rust/)
• Add Rust Explorer (https://www.rustexplorer.com/)
• Add Rustfinity (https://www.rustfinity.com/playground)

All entries properly formatted and placed in alphabetical order within the Rust section.

What does this PR do?

Add resource(s) | Remove resource(s) | Add info | Improve repo

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For resources

Description

Why is this valuable (or not)?

How do we know it's really free?

For book lists, is it a book? For course lists, is it a course? etc.

Checklist:

• Search for duplicates.
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction).
• Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!

[github-actions]

Linter failed, fix the error(s):

free-programming-books/more/free-programming-playgrounds.md
395:3-395:49  warning  Remove trailing slash (https://www.rustexplorer.com)  no-url-trailing-slash  remark-lint

[Tejas-Santosh-Nalawade]

@eshellman Please Review it and merge tell if any changes