chore: add codespaces badge

[dreamorosi]

Description of your changes

As described in #1154, GitHub announced a free tier for GitHub CodeSpaces. This PR adds a one-click launch button in the Contributing docs so that prospective contributors can get started easily.

How to verify this change

See rendered CONTRIBUTING docs, optionally test the setup (it's free).

Related issues, RFCs

Issue number: #1154

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published in downstream module
• The PR title follows the conventional commit semantics

Breaking change checklist

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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