Thanks for the post. That is a very old document that we will update or remove. It does give the url when you click the "Arduino IDE Board Manager" link near the top of the document. The document was produced before the esp32 team made an Arduino compatible package and when the concept of a manager was first introduced by Arduino some years ago. The url the document provides should be ignored because there are other more recent packages you could use instead.

There is more than one package for the Esp32 boards. The point to bear in mind with Visual Micro is Arduino IDE compatible, it uses the same configuration. Any install doc for the esp32 and the arduino ide would result in esp32 being installed in Visual Micro. We don't maintain the urls, you have to find the instructions for whatever Arduino comaptible board you have purchased.

We do also have guides for the most popular boards, of which esp32 is one of them. If you use the "Docs" links from the top of visualmicro.com, it shows our documentatio page. You will see the Esp32 overview link and that does give an example json url.
https://www.visualmicro.com/page/User-Guide.aspx

The Board Manager in Visual Micro provides similar functionality to the Board Manager in the Arduino IDE. It is worth reading up about how the Board Manager works. 

In Visual Micro,, at the top of the Board Manager packages tree you will see "Auto-Discovery", that provides a list of package that have been added to the officual arduino.cc "unofficial packages" list. Esp board will be listed there too. The Auto-Discovery list removes the need for you to know many standard urls, double click to automatically add the corresponding url.

+ The Esp team have multiple branches each with a different url. You can read about them on their web site along with their guides of how to install into Arduino.

I guess you have been using the built in AVR of the Arduino IDE for the past years and the Board Manager is a new concept? Or have I misunderstood the question?