Rename live demo to playground

[khanhtc1202]

What this PR does:

Rename demo related docs to playground to make it less confusing.

Why we need it:

We have play.pipecd.dev mentioned as live demo, while demo.pipecd.dev used as dev. Should rename to make it less confusing 👀

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.16%. Comparing base (fdb9066) to head (375b7af).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5974   +/-   ##
=======================================
  Coverage   28.16%   28.16%           
=======================================
  Files         516      516           
  Lines       55895    55895           
=======================================
  Hits        15743    15743           
  Misses      38900    38900           
  Partials     1252     1252           

Flag	Coverage Δ
.	23.22% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	50.89% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[ffjlabo]

👍