feat: add emit events and update spec docs

[jaybxyz]

Description

closes: #38

Tasks

• Add emit events and update 05_events.md
• Update other spec docs

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• Appropriate labels applied
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• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
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• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #46 (3e670ef) into master (b1fd562) will increase coverage by 0.68%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
+ Coverage   79.85%   80.54%   +0.68%     
==========================================
  Files          12       12              
  Lines         283      293      +10     
==========================================
+ Hits          226      236      +10     
  Misses         42       42              
  Partials       15       15              

Impacted Files	Coverage Δ
x/budget/keeper/budget.go	82.02% <100.00%> (+2.27%)	⬆️

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[dongsam]

I suggested some lines, could you review the suggestions?

[dongsam]

LGTM