Remove custom intersphinx module from `docs/_ext`

[phlax]

In order to workaround sphinx-doc/sphinx#2068 a custom intersphinx module was added to docs/_ext

There is an open PR to fix this upstream here sphinx-doc/sphinx#8981, which is currently scheduled to land as part of sphinx==4.1

Once upstream is fixed we will want to remove the custom intersphinx

We may also need to update the module when we upgrade to sphinx==4

[github-actions]

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