docs: list MongoDB and shared sandbox examples

[AbdulSamad94]

This pull request updates the examples gallery to include two released examples that were missing from their corresponding categories:

• MongoDB-backed session storage under the memory examples
• A shared sandbox session with separate SandboxRunConfig.cwd values under the sandbox examples

Both examples are available in v0.22.0. This change only improves their discoverability and does not modify SDK behavior.

Test plan

• Confirmed both referenced example paths exist
• Performed a focused Japanese, Korean, and Chinese translation-safety review of the added English text
• Ran git diff --check
• Ran the documentation reference generator
• Ran mkdocs build successfully