Azure pipeline activation

Nobody has claimed this yet.

I have probably forgotten some steps, but this is mostly all you need to know:

1. Sign up for [Azure DevOps](https://dev.azure.com/).
2. Create an organization. Mine is my username, but you can probably think of something cooler.
3. Create a new public project associated with the organization. I took the easy route and named mine 'pynndescent', but you can have multiple repos associated with a project (e.g. UMAP could go under here too eventually), so something more generic might be more appropriate.
4. On the left set of icons, click on 'Pipeline'. In the pane that appears click the 'New' button, then 'New build pipeline'. Select github from the list of providers and then this repo (obviously).
5. If you haven't already been prompted to, you will be told that you need to install the [Azure Pipelines GitHub App](https://go.microsoft.com/fwlink/?linkid=874367) for this repo. This provides the typical read/write permissions.
6. The existing `azure-pipelines.yml` file in this repo will be picked up. Once permissions from the previous step have been granted, you can click the 'Run' button on the top right to kick it off.
7. That should be it. You can see the history of the builds if the current pipeline is selected on the main pipeline page. Also on the very top right is the context menu (the easy-to-miss three vertical dots under which everything useful is hidden according to the diktats of modern UI design). In there, under 'Status badge' is the URL for the all-important README badge.

Start with the existing azure-pipelines.yml file and the repository's Azure DevOps pipeline setup. Follow the documented Azure Pipelines GitHub integration steps, then run the pipeline and verify that a build completes successfully and the status badge URL is available.

Written by the indexing model from the issue text.