Point people to READTHEDOCS for help, not GitHub

Most users w/the Library Manager will get a little nervous with the GH site.
The ReadTheDocs site includes links to the GH site, too, so they can still
find the origin.

Author: earlephilhower

package/package_pico_index.template.json

@@ -6,7 +6,7 @@
          "websiteURL": "https://github.com/earlephilhower/arduino-pico",
          "email": "[email protected]",
          "help": {
-            "online": "https://github.com/earlephilhower/arduino-pico"
+            "online": "https://arduino-pico.readthedocs.io/en/latest/"
          },
          "platforms": [
             {