docs: Add demo link to README.md

[alexec]

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[codecov]

Codecov Report

Merging #5512 (7084bb9) into master (bbdf651) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 7084bb9 differs from pull request most recent head e57a7d3. Consider uploading reports for the commit e57a7d3 to get more accurate results

@@            Coverage Diff             @@
##           master    #5512      +/-   ##
==========================================
- Coverage   46.78%   46.74%   -0.04%     
==========================================
  Files         240      240              
  Lines       15004    15004              
==========================================
- Hits         7019     7014       -5     
- Misses       7087     7091       +4     
- Partials      898      899       +1     

Impacted Files	Coverage Δ
cmd/argo/commands/get.go	56.00% <0.00%> (-1.34%)	⬇️
workflow/controller/operator.go	70.69% <0.00%> (-0.28%)	⬇️
cmd/argoexec/commands/emissary.go	50.00% <0.00%> (+1.56%)	⬆️
workflow/metrics/server.go	16.66% <0.00%> (+4.16%)	⬆️

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