[Refactor] Create doc.go for each package

[vankichi]

Describe the bug:

In the Vald project, almost packages have no doc.go.
It affects displaying on the pkg.go.dev, e.g. https://pkg.go.dev/github.com/vdaas/[email protected]/internal/compress .

To Reproduce:

None (This problem does NOT affect the Vald)

Expected behavior:

Displaying correct state like as https://pkg.go.dev/github.com/vdaas/[email protected]/internal/errgroup
We can fix it:

1. Add doc.go for each package
2. Remove the overview comment from each file except doc.go

Environment:

• Go Version: 1.19.2
• Docker Version: 20.10.8
• Kubernetes Version: 1.22.0
• NGT Version: 1.14.8