Document how Overlay works behind the scenes #18660

follows up from https://github.com/matomo-org/matomo/issues/18538

We will want to document on https://developer.matomo.org/piwik-in-depth how the Overlay feature works. I would have mentioned this as part of #18538 but better to have a separate issue for it.

It be great to document the flow like

• Overlay is opened
• The website is embedded as an iframe
  • The website within the frame calls ... to ...
  • This calls ... to ...

We want to document how the iframe and the parent frame communicates with each other.

We also want to cover, possibly as part of the flow, what some of these actions like notifyParentIframe and startOverlaySession do.