Add user-friendly landing page to DEA Sandbox

[robbibt]

Proposed changes

This PR provides a user friendly landing page that will make it easier for users to learn about the content available in the Sandbox, and useful reference information in the Knowledge Hub:

Closes issues (optional)

• Closes Issue #1188

Checklist

(Replace [ ] with [x] to check off)

• [ ] Notebook created using the DEA-notebooks template
• [ ] Remove any unused Python packages from Load packages
• [ ] Remove any unused/empty code cells
• [ ] Remove any guidance cells (e.g. General advice)
• [ ] Ensure that all code cells follow the PEP8 standard for code. The jupyterlab_code_formatter tool can be used to format code cells to a consistent style: select each code cell, then click Edit and then one of the Apply X Formatter options (YAPF or Black are recommended).
• [ ] Include relevant tags in the final notebook cell (refer to the DEA Tags Index, and re-use tags if possible)
• [x] Clear all outputs, run notebook from start to finish, and save the notebook in the state where all cells have been sequentially evaluated
• [ ] Test notebook on both the NCI and DEA Sandbox (flag if not working as part of PR and ask for help to solve if needed)
• [ ] If applicable, update the Notebook currently compatible with the NCI|DEA Sandbox environment only line below the notebook title to reflect the environments the notebook is compatible with
• [x] Check for any spelling mistakes using the DEA Sandbox's built-in spellchecker (double click on markdown cells then right-click on pink highlighted words). For example:

[review-notebook-app[bot]]

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