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 69.66%. Comparing base (4b8eb28) to head (4835844).
⚠️ Report is 192 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #540      +/-   ##
============================================
- Coverage     71.30%   69.66%   -1.65%     
- Complexity     1275     1322      +47     
============================================
  Files            46       46              
  Lines          3792     3975     +183     
============================================
+ Hits           2704     2769      +65     
- Misses         1088     1206     +118     

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