ci(publish_site): fix workflow

[Warashi]

What this PR does:

• Use npm ci to install dependencies in the CI
• Ran npm audit fix in the docs directory

Why we need it:

to address the workflow failure
https://github.com/pipe-cd/pipecd/actions/runs/23999586973/job/69993362383#step:6:65

Which issue(s) this PR fixes:

I haven't opened any issue, because this is a small, simple fix.

Does this PR introduce a user-facing change?: No

• 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 29.47%. Comparing base (6e2287e) to head (8fbd367).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6647   +/-   ##
=======================================
  Coverage   29.47%   29.47%           
=======================================
  Files         584      584           
  Lines       62474    62474           
=======================================
  Hits        18414    18414           
  Misses      42636    42636           
  Partials     1424     1424           

Flag	Coverage Δ
.	23.29% <ø> (ø)
.-pkg-app-pipedv1-plugin-analysis	32.43% <ø> (ø)
.-pkg-app-pipedv1-plugin-ecs	30.84% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	58.37% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	63.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	37.95% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	33.04% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	52.71% <ø> (ø)
.-pkg-plugin-sdk	50.34% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.51% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[eeshaanSA]

LGTM, thanks @Warashi