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

app/plugin_commands.go

@@ -105,7 +105,7 @@ func (a *App) ExecutePluginCommand(args *model.CommandArgs) (*model.Command, *mo
 			if err != nil {
 				return pc.Command, nil, model.NewAppError("ExecutePluginCommand", "model.plugin_command.error.app_error", nil, "err="+err.Error(), http.StatusInternalServerError)
 			}
-			response, appErr := pluginHooks.ExecuteCommand(plugin.NewBlankContext(), args)
+			response, appErr := pluginHooks.ExecuteCommand(&plugin.Context{}, args)
 			return pc.Command, response, appErr
 		}
 	}