Fix sidepanel contents for section on noise

[mhucka]

The removal of some documents in a previous PR left the left-side TOC panel with an extra heading. Now, it also turns out that none of the other sections in the Cirq docs use an accordion, so we can feed two birds with one scone by getting rid of the accordion and moving the contents up one level. This should result in a properly-structured panel.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (3031b1f) to head (a159be0).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7518   +/-   ##
=======================================
  Coverage   98.68%   98.68%           
=======================================
  Files        1093     1093           
  Lines       97112    97112           
=======================================
+ Hits        95837    95838    +1     
+ Misses       1275     1274    -1     

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