docs: Refactor extensions into packages, update the add-ons-reference docs page

Includes converting some of the remaining standalone scripts into Sphinx extensions.

Make flake8 clean

docs/idf_extensions/esp_err_definitions.py

@@ -0,0 +1,14 @@
+# Extension to generate esp_err definition as .rst
+from util import copy_if_modified, call_with_python
+
+
+def setup(app):
+    app.connect('idf-info', generate_err_defs)
+    return {'parallel_read_safe': True, 'parallel_write_safe': True, 'version': '0.1'}
+
+
+def generate_err_defs(app, project_description):
+    # Generate 'esp_err_defs.inc' file with ESP_ERR_ error code definitions from inc file
+    esp_err_inc_path = '{}/inc/esp_err_defs.inc'.format(app.config.build_dir)
+    call_with_python('{}/tools/gen_esp_err_to_name.py --rst_output {}.in'.format(app.config.idf_path, esp_err_inc_path))
+    copy_if_modified(esp_err_inc_path + '.in', esp_err_inc_path)