Creating pipelines zip archive in docs CI on every push to a PR branch

I noticed that there's an artifact getting uploaded as part of the docs build CI even if the CI wasn't triggered by a tag push:

https://github.com/DiamondLightSource/httomo/blob/899a43da8c63a1431d0804dcb48773be08b46710/.github/workflows/httomo_docs.yml#L43-L45

https://github.com/DiamondLightSource/httomo/blob/899a43da8c63a1431d0804dcb48773be08b46710/.github/workflows/httomo_docs.yml#L56-L60

The artifact isn't very big (taken from https://github.com/DiamondLightSource/httomo/actions/runs/24771337928/job/72478300291?pr=708):
```
With the provided path, there will be 1 file uploaded
Artifact name is valid!
Root directory input is valid!
Beginning upload of artifact content to blob storage
Uploaded bytes 16937
Finished uploading artifact content to blob storage!
SHA256 digest of uploaded artifact zip is 5e9dd090dcaad7c024f063b92dcd1c1b6e945ce80a57d2491132ba2461fd364a
Finalizing artifact upload
Artifact full_pipelines-artifact.zip successfully finalized. Artifact ID 6575004505
Artifact full_pipelines-artifact has been successfully uploaded! Final size is 16937 bytes. Artifact ID is 6575004505
Artifact download URL: https://github.com/DiamondLightSource/httomo/actions/runs/24771337928/artifacts/6575004505
```

but if this happens every time a push is done to a PR branch then this could accumulate, and I imagine it'd accumulate somewhere associated with the repo and it'll eventually reach a storage limit of some kind.

We probably want to only create this artifact for releases, which would be equivalent to something like the following condition used for the docs upload, just without the `main` part: https://github.com/DiamondLightSource/httomo/blob/899a43da8c63a1431d0804dcb48773be08b46710/.github/workflows/httomo_docs.yml#L51