Improve the documentation for connecting the plugin from the frontend

[hayhaydz]

Hey,

I've been implementing this plugin into a site and came across an issue in that I had to go digging in order to work out what kind of POST request was needed to submit data into a form submission. It'd be great if it was shown somewhere in the documentation. Now that I know it seems very simple but at the time I was a little confused.

From what I've worked out by looking here (https://github.com/payloadcms/website/blob/main/src/components/CMSForm/index.tsx) is that the request looks roughly like this:

const req = await fetch(`${import.meta.env.PUBLIC_SITE_URL}/api/contact`, {
      method: 'POST',
      headers: {
          'Content-Type': 'application/json',
      },
      body: JSON.stringify({
          form: formID,
          submissionData: formdata,
      }),
  });