MM-11292: clean up plugins GoDoc (#9109)

* clean up plugins GoDoc:

- eliminate plugin.NewBlankContext() as unnecessary
- export ValidIdRegex as a string vs. the less readable var
- add/update various documentation strings
- hide everything by default, except where used by client plugins or the mattermost-server. The exception to this rule are the `*(Args|Returns)` structs which must be public for go-plugin, but are now prefixed with `Z_` with a warning not to use.
- include a top-level example to get plugin authors started

This is not a breaking change for existing plugins compiled against
plugins-v2.

* remove commented out ServeHTTPResponseWriter

* update examples to match developer docs

* add missing plugin/doc.go license header

plugin/example_hello_world_test.go

@@ -0,0 +1,20 @@
+package plugin_test
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/plugin"
+)
+
+type HelloWorldPlugin struct{}
+
+func (p *HelloWorldPlugin) ServeHTTP(c *plugin.Context, w http.ResponseWriter, r *http.Request) {
+	fmt.Fprintf(w, "Hello, world!")
+}
+
+// This example demonstrates a plugin that handles HTTP requests which respond by greeting the
+// world.
+func Example_helloWorld() {
+	plugin.ClientMain(&HelloWorldPlugin{})
+}