docs: missing references in `load_3d_dataset.rst`

[pjonsson]

I've fixed a bunch of Sphinx warnings, but there are still two missing references in load_3d_dataset.rst:

/code/docs/ops/load_3d_dataset.rst:10: WARNING: undefined label: 'extending-datacube-3d-reads' [ref.ref]
/code/docs/ops/load_3d_dataset.rst:10: WARNING: undefined label: 'product-doc-extra-dim' [ref.ref]

[SpacemanPaul]

The functionality described in this section is basically replaced by the multi-dimensional loading capabilities from odc-loader. It's really only included at this point as a historical footnote.

[pjonsson]

ReadTheDocs has a version selector, can the entire file be replaced with a link to odc-loader in the 1.9 branch?

Less text in the documentation is better for everyone, less to read for the user and less to maintain for the developers.