Update documentation to install jupyterlite-xeus via conda

[mcbarton]

Description

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

This PR https://github.com/compiler-research/xeus-cpp/pull/330/files missed the documentation changes, and created an inconsistency between the main workflow and the deployment workflow. Currently the deployment workflow has jupyterlite-core pinned to 0.6 , but the main workflow was unpinned. I made them both pinned, but will make both unpinned if told otherwise.

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 81.78%. Comparing base (ca05e91) to head (7904315).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #332   +/-   ##
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  Coverage   81.78%   81.78%           
=======================================
  Files          20       20           
  Lines         950      950           
  Branches       87       87           
=======================================
  Hits          777      777           
  Misses        173      173           

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