Docs: Clean up old documentation and create new documentation related to the variant upload script

- [ ] With the removal of the staging and PoC instances some of the documentation is now out-dated and should be removed. Might leave this in for now since the staging instance should be reinstated relatively soon.
- [ ] With the first batch of variants being uploaded to G4RD PT, documentation should be added for the variant pre-processing and POSTing script.

Locate the documentation covering the staging and PoC instances and the variant pre-processing and POSTing script. Check which staging material is still relevant, then document the script’s use for uploading the first batch of variants to G4RD PT. Done means outdated material is removed or retained with a clear reason, and the upload workflow is documented.

Written by the indexing model from the issue text.