feat: create a Go sample for the docs first plugin

[walves-cit]

This plugin is a Go version from the already implemented in C++ and Rust docs_first_plugin.

This examples contains only a Go version.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• plugins/samples/docs_first_plugin/plugin.go:15, tag serviceextensions_plugin_docs_first_plugin

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