Update docs to introduce code-server extension

[c-bata]

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This pull request may therefore be ported to Goptuna.
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Reference Issues/PRs

Follow-up #741

What does this implement/fix? Explain your changes.

Update docs to introduce code-server extension.

[c-bata]

@HideakiImamura Could you review this PR?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5c2bc4a) 68.20% compared to head (0997b61) 68.20%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #750   +/-   ##
=======================================
  Coverage   68.20%   68.20%           
=======================================
  Files          35       35           
  Lines        2334     2334           
=======================================
  Hits         1592     1592           
  Misses        742      742           

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[HideakiImamura]

LGTM.