docs: repoint jupyterlite xeus channel to prefix.dev

The emscripten-forge channel host repo.mamba.pm has been
decommissioned, so `jupyter lite build` can no longer fetch its
repodata and every Read the Docs build fails at the 'try dpdata
online' step. Point the in-browser kernel environment at the current
emscripten-forge channel on prefix.dev; monty and wcmatch continue to
resolve from conda-forge noarch.

Author: Han Wang

docs/environment.yml

@@ -1,6 +1,6 @@
 name: dpdata
 channels:
-  - https://repo.mamba.pm/emscripten-forge
+  - https://prefix.dev/emscripten-forge-dev
   - conda-forge
 dependencies:
   - xeus-python