Unhide section toc on mobile

[agoujot]

And I think this is better than a dropdown, as that would make two dropdowns on either side; plus, it would be less accessible to users.

Bug: T177900

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.69%. Comparing base (4b8eb28) to head (b486ad9).
⚠️ Report is 256 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##               main     #540       +/-   ##
=============================================
- Coverage     71.30%   60.69%   -10.62%     
- Complexity     1275     1409      +134     
=============================================
  Files            46       46               
  Lines          3792     4320      +528     
=============================================
- Hits           2704     2622       -82     
- Misses         1088     1698      +610     

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