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>

src/locales/de/translation.json

@@ -176,8 +176,11 @@
   "Error:": "Fehler:",
   "MapTiler Access Token": "MapTiler Zugriffstoken",
   "Public access token for MapTiler Cloud.": "Öffentlicher Zugriffstoken für MapTiler Cloud.",
+  "Learn More": "Mehr erfahren",
   "Thunderforest Access Token": "Thunderforest Zugriffstoken",
   "Public access token for Thunderforest services.": "Öffentlicher Zugriffstoken für Thunderforest-Dienste.",
+  "Stadia Maps API Key": "Stadia Maps API-Schlüssel",
+  "API key for Stadia Maps.": "API-Schlüssel für Stadia Maps.",
   "Style Renderer": "Stil-Renderer",
   "Choose the default Maputnik renderer for this style.": "Wähle den Standard-Renderer für diesen Stil aus.",
   "Language": "Sprache",