Update URL for cyipopt documentation

[jsiirola]

Fixes # .

Summary/Motivation:

The Cyipopt documentation appears to have dropped the /en/ from the RTD documentation pages. This tracks that change.

Changes proposed in this PR:

• Update cyipopt RTD URLs

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.93%. Comparing base (e5df38e) to head (3181395).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3909   +/-   ##
=======================================
  Coverage   89.93%   89.93%           
=======================================
  Files         902      902           
  Lines      106415   106415           
=======================================
+ Hits        95701    95702    +1     
+ Misses      10714    10713    -1     

Flag	Coverage Δ
builders	29.18% <ø> (-0.01%)	⬇️
default	86.23% <ø> (?)
expensive	35.63% <ø> (?)
linux	87.38% <ø> (-2.04%)	⬇️
linux_other	87.38% <ø> (ø)
oldsolvers	28.10% <ø> (+<0.01%)	⬆️
osx	82.71% <ø> (ø)
win	85.60% <ø> (-0.20%)	⬇️
win_other	85.60% <ø> (-0.20%)	⬇️

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