chore: Add menu-item-checkbox example

[leonardoelias]

Adds a "Menu item checkbox" example #939.

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Codecov Report

Merging #961 (f9f65ff) into v2 (f950771) will increase coverage by 12.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##               v2     #961       +/-   ##
===========================================
+ Coverage   42.15%   54.15%   +12.00%     
===========================================
  Files         155      156        +1     
  Lines        4109     4114        +5     
  Branches     1158     1159        +1     
===========================================
+ Hits         1732     2228      +496     
+ Misses       2376     1885      -491     
  Partials        1        1               

Impacted Files	Coverage Δ
...src/menu/__examples__/menu-item-checkbox/index.tsx	100.00% <100.00%> (ø)
packages/ariakit/src/composite/composite.ts	91.11% <0.00%> (+0.74%)	⬆️
packages/ariakit/src/composite/composite-item.tsx	59.45% <0.00%> (+1.62%)	⬆️
packages/ariakit/src/composite/composite-state.ts	86.45% <0.00%> (+7.29%)	⬆️
packages/ariakit/src/menu/menu-item-radio.tsx	9.09% <0.00%> (+9.09%)	⬆️
packages/ariakit/src/dialog/dialog.tsx	66.87% <0.00%> (+12.50%)	⬆️
packages/ariakit/src/checkbox/checkbox.tsx	84.28% <0.00%> (+12.85%)	⬆️
packages/ariakit/src/menu/menu-bar.ts	22.22% <0.00%> (+22.22%)	⬆️
packages/ariakit/src/menu/menu-group-label.ts	28.57% <0.00%> (+28.57%)	⬆️
packages/ariakit/src/menu/menu-group.ts	33.33% <0.00%> (+33.33%)	⬆️
... and 22 more

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[saideepesh000]

hey @leonardoelias , can u also add a readme file for menu, something like this Button

[diegohaz]

❤️