 CI/DOC: Building the documentation with azure-pipelines
- [X] xref #22766, #26574
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Looks like the problem in #26591 was that the `HostName` input of the azure task that installs the key, is not the host name, but the `know_hosts` line for the host name where to push.

I set up azure-pipelines in my pandas branch, to be able to see the `master` builds and avoid surprises, and this branch is successfully pushing the docs to https://dev.pandas.io

@TomAugspurger since there were differences between the travis docs dependencies and `environment.yml` I think it makes more sense to change and use `environment.yml` here, so we can detect and fix any problem while we still have the travis docs, and then simply remove the travis docs dependencies with the build when we are happy with this.

@jorisvandenbossche the `ImportError` for `pyarrow` doesn't seem to be because `pyarrow` is not in `environment.yml`, it is. There is something weird with that, need to have a look.