Clarify CodeSandbox works for private projects

The sentence about the Open in CodeSandbox button was causing confusion and leading people to believe that the feature only works for public projects.

This change splits the para in two to make it less ambiguous.

Related issues

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/47268

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards