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/libs/field-spec-additional.ts

@@ -4,15 +4,25 @@ const spec = (t: TFunction) => ({
   maputnik: {
     maptiler_access_token: {
       label: t("MapTiler Access Token"),
-      doc: t("Public access token for MapTiler Cloud.")
+      doc: t("Public access token for MapTiler Cloud."),
+      docUrl: "https://docs.maptiler.com/cloud/api/authentication-key/",
+      docUrlLinkText: t("Learn More")
     },
     thunderforest_access_token: {
       label: t("Thunderforest Access Token"),
-      doc: t("Public access token for Thunderforest services.")
+      doc: t("Public access token for Thunderforest services."),
+      docUrl: "https://www.thunderforest.com/docs/apikeys/",
+      docUrlLinkText: t("Learn More")
+    },
+    stadia_access_token: {
+      label: t("Stadia Maps API Key"),
+      doc: t("API key for Stadia Maps."),
+      docUrl: "https://docs.stadiamaps.com/authentication/",
+      docUrlLinkText: t("Learn More")
     },
     style_renderer: {
       label: t("Style Renderer"),
-      doc: t("Choose the default Maputnik renderer for this style."),
+      doc: t("Choose the default Maputnik renderer for this style.")
     },
   }
 })