auto generate plugin docs directory

[onecer]

Is your feature request related to a problem? Please describe.

When I try to gen doc, got this error

$ go run cmd/doc-gen/main.go
Error while generating documentation: open ./docs/plugins/fluentbit/multilineparser/multilineparser.md: no such file or directory

if relate dirs is not exists , gen doc will failed. like multilineparser plugin eg. ./docs/plugins/fluentbit/multilineparser

Describe the solution you'd like

it should be auto created directory when it does not exist.

people don't need to create it manually.

Additional context

No response