Add links to tile provider authentication docs (#914)

This probably confused some people in the past, since vector tiles won't
even display an access denied image ;)

Before (no information on access keys and where to get them):

<img width="411" alt="image"
src="https://github.com/user-attachments/assets/8820fb20-bda4-460c-9cc9-8fec5daa480d">

After (add links to providers in info callout + add a field for Stadia
Maps API keys):

<img width="395" alt="image"
src="https://github.com/user-attachments/assets/91ee732c-b3f5-45f8-81a6-8d896a93f644">

---------

Co-authored-by: Harel M <harel.mazor@gmail.com>
Co-authored-by: Joscha <34318751+josxha@users.noreply.github.com>
Co-authored-by: Hugues Tavernier <hugues.tavernier@protonmail.com>
Co-authored-by: Keitaroh Kobayashi <keita@kbys.me>

cypress/e2e/modals.cy.ts

@@ -160,6 +160,18 @@ describe("modals", () => {
       ).shouldInclude({ "maputnik:thunderforest_access_token": apiKey });
     });
 
+    it("stadia access token", () => {
+      let apiKey = "testing123";
+      when.setValue(
+        "modal:settings.maputnik:stadia_access_token",
+        apiKey
+      );
+      when.click("modal:settings.name");
+      then(
+        get.styleFromLocalStorage().then((style) => style.metadata)
+      ).shouldInclude({ "maputnik:stadia_access_token": apiKey });
+    });
+
     it("style renderer", () => {
       cy.on("uncaught:exception", () => false); // this is due to the fact that this is an invalid style for openlayers
       when.select("modal:settings.maputnik:renderer", "ol");