Adding the ability to inject websites directly inside maps

This PR adds the ability to inject a website INSIDE a map (as an iframe inside a Phaser HTML object)
The iFrame will be rendered transparently, unless you set a background-color on the body, which opens a number of cool possibilities.

Needs to be done: allowing the iframe API in those iframes.

docs/maps/website-in-map.md

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+{.section-title.accent.text-primary}
+# Putting a website inside a map
+
+You can inject a website directly into your map, at a given position.
+
+To do this in Tiled:
+
+- Select an object layer
+- Create a rectangular object, at the position where you want your website to appear
+- Add a `url` property to your object pointing to the URL you want to open
+
+<div>
+    <figure class="figure">
+        <img src="https://workadventu.re/img/docs/website_url_property.png" class="figure-img img-fluid rounded" alt="" style="width: 70%" />
+        <figcaption class="figure-caption">A "website" object</figcaption>
+    </figure>
+</div>
+
+The `url` can be absolute, or relative to your map.
+
+{.alert.alert-info}
+Internally, WorkAdventure will create an "iFrame" to load the website.
+Some websites forbid being opened by iframes using the [`X-Frame-Options](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options)
+HTTP header.