Add example notebooks + readme.md to Jupyter Lite demo

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

After the Jupyter Lite xeus-cpp demo sucessfully deployed as a Github page I realised that the demo was missing the example notebook and README.md . @vgvassilev @anutosh491 can one of you merge this PR to add these to the demo?

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.05%. Comparing base (d9eff39) to head (ffd3493).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #146   +/-   ##
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  Coverage   82.05%   82.05%           
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  Files          17       17           
  Lines         613      613           
  Branches       59       59           
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  Hits          503      503           
  Misses        110      110