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_requests.go

@@ -72,5 +72,5 @@ func (a *App) servePluginRequest(w http.ResponseWriter, r *http.Request, handler
 	r.URL.RawQuery = newQuery.Encode()
 	r.URL.Path = strings.TrimPrefix(r.URL.Path, "/plugins/"+params["plugin_id"])
 
-	handler(plugin.NewBlankContext(), w, r)
+	handler(&plugin.Context{}, w, r)
 }