Trying to code a simple plugin, infinite loop?

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Take a closer look at add_settings_field(); your callback is the same as of add_options_page().

You’ve already set up your input function. Just change site_quotes_output_settings to site_quotes_main_create_input and everything should be fine:

add_settings_field(
  'sometext',
  'Some text!',
  'site_quotes_main_create_input',
  'site_quotes_plugin',
  'site_quotes_settings_section'
);