Fix invalid schema for jupyterlite_sphinx_strip tag when strip_tagged_cells is True

[agriyakhetarpal]

Description

"metadata": {
        "tags": [
          "jupyterlite_sphinx_strip": "true"
        ]
      }

is invalid for notebook-based metadata, while "jupyterlite_sphinx_strip" is valid. This was incorrectly mentioned in the documentation and was not caught because we were trying to parse the cell metadata itself. We now check for the existence of a tag in the metadata rather than trying to find it as a key-value pair in the metadata for a cell. This is a fix that is a candidate for a v0.16.1 release.

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cc: @steppi

[steppi]

Looks good @agriyakhetarpal. Thanks for catching this early.