Restore inline cppreference documentation

[anutosh491]

Description

Fixes #484

Restore inline cppreference documentation for xeus-cpp inspection requests :

cppreference now uses canonical URLs under /cpp/... instead of /w/cpp/....

Additionally, xeus-cpp was appending ?action=purge to the documentation URL before embedding it in an iframe. Although the regular cppreference page can be displayed in an iframe, the purge request returns:

X-Frame-Options: DENY

The browser therefore refuses to display the documentation, leaving the inspect output blank.

Type of change

• 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 72.84%. Comparing base (3ead278) to head (471c88e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #495   +/-   ##
=======================================
  Coverage   72.84%   72.84%           
=======================================
  Files          18       18           
  Lines        1204     1204           
  Branches      113      113           
=======================================
  Hits          877      877           
  Misses        327      327           

Files with missing lines	Coverage Δ
src/xinspect.cpp	91.81% <100.00%> (ø)

Files with missing lines	Coverage Δ
src/xinspect.cpp	91.81% <100.00%> (ø)

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[github-actions]

clang-tidy review says "All clean, LGTM! 👍"