FIX: resolve JupyterLite notebook install errors in CircleCI preview

Two targeted fixes to doc/conf.py:

1. Clean dist_lite_dir before each wheel build so stale wheels from
   previous runs do not accumulate in the piplite all.json index.
   Previously each Sphinx build appended a new mne-9999.x.x wheel to
   the directory, causing all.json to grow across runs and contain
   multiple stale wheel entries.

2. Remove the hard version pin 'mne==9999.0.1' from the
   first_notebook_cell piplite.install call. The version 9999.0.1 is a
   local dev build that only exists in the generated all.json index.
   When the CircleCI artifact viewer loads a notebook, the Pyodide
   Web Worker cannot fetch the local all.json because CircleCI's
   authentication proxy blocks unauthenticated cross-origin requests
   from sandboxed Web Workers. piplite then silently falls back to
   public PyPI, where mne==9999.0.1 does not exist, causing a
   ValueError in every intro notebook.

   Removing the pin allows piplite to install the highest available
   version: the local dev wheel when all.json is reachable (production
   docs server), and the latest stable MNE release from PyPI when it is
   not (CircleCI preview). Both paths result in a working notebook
   instead of a traceback.

Author: natinew77-creator

doc/conf.py

@@ -534,6 +534,9 @@
 dist_lite_dir = os.path.join(
     os.path.abspath(os.path.dirname(__file__)), "_build", "dist_lite"
 )
+# Clean the directory first so stale wheels from previous runs do not
+# accumulate and pollute the piplite all.json index.
+shutil.rmtree(dist_lite_dir, ignore_errors=True)
 os.makedirs(dist_lite_dir, exist_ok=True)
 
 pyproject_path = os.path.join(os.path.dirname(__file__), "..", "pyproject.toml")
@@ -565,7 +568,12 @@
     "first_notebook_cell": (
         "# 💡 This cell is automatically added to the start of each notebook.\n"
         "import piplite\n"
-        "await piplite.install(['mne==9999.0.1', 'scikit-learn', 'joblib'])\n"
+        "# Install mne without a version pin so piplite resolves the local\n"
+        "# dev wheel from all.json when available, and falls back cleanly to\n"
+        "# the latest stable release from PyPI in restricted preview\n"
+        "# environments (e.g. CircleCI artifact viewer) where the local index\n"
+        "# is blocked by the host's authentication proxy.\n"
+        "await piplite.install(['mne', 'scikit-learn', 'joblib'])\n"
         "\n"
         "# Mock lzma since it is missing in Pyodide but required by pooch/joblib\n"
         "import sys\n"