Add a link to test deployment to PRs

[NickVolynkin]

When deploying from a branch, add a link to the test deployment, just as it is in tarantool/doc.

For example, in tarantool/doc#3022:

Use single deployment called 'test', just like the directory in the S3 bucket.

https://github.com/tarantool/doc/blob/latest/.github/workflows/deploy-branch.yml#L28-L35

After building and uploading docs,

https://github.com/tarantool/doc/blob/latest/.github/workflows/deploy-branch.yml#L44-L51