docs(env-variables): Add cURL example for refresh endpoint

[riddla]

Description

I added a cURL example for the refresh endpoint e.g. http://localhost:8000/__refresh.

Related Issues

Related to #10328.

[DSchau]

Left a small comment; not sure if it's worth documenting but we want to avoid confusion wherever we can.

I'll leave it up to you!

[riddla]

Thanks for the addition. Makes totally sense.

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[riddla]

@DSchau, I was wondering if a small helper component like the following would/could make sense, also for others:

import React from 'react'

let style = {
  // ...
}

const RefreshContent = ({ children }) => (
  <button onClick={refresh} style={style}>
    Refresh content
  </button>
)

function refresh() {
  console.log('refreshing content')
  var oReq = new XMLHttpRequest()
  oReq.open('POST', 'http://localhost:8000/__refresh')
  oReq.send()
}

export default RefreshContent

As I'm new to the Gatsby ecosystem I don't know if those kind of (development) helper components already exist, where I could put them etc.
Maybe you have some insights?