Changed the link to old pytket-cutensornet docs

[PabloAndresCQ]

Description

Changing the link for the docs of pytket-cutensornet to the old version of the docs (which is still maintained for pytket-cutensornet) since there are some problems with the new docs (see issue linked below).

NOTE: This is a temporary solution.

Related issues

CQCL/pytket-cutensornet#139

Checklist

• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the changelog with any user-facing changes.

[cqc-melf]

@PabloAndresCQ for this to take full effect, you need to update this on the docmain branch, too.