Dead link to docs in repo About section

[TSGreen]

There is URL in the repository "About" section which links to the documentation build for this project. The URL is still pointed towards the former location of this repository which no longer exists.

The GitHub Action to build docs is still active, so the documentation has been rebuilt at the new location. This URL should therefore be changed from https://ons-sst.github.io/cis_households/ to https://onsdigital.github.io/cis_households/