ENH: branch deployments of documentation
#### Is your feature request related to a problem?

When submitting large/complex pull requests around documentation, I've been (reasonably) asked to submit a screenshot of the change. These are tedious to capture, the links in screenshots aren't clickable, the text isn't searchable, etc.

#### Describe the solution you'd like

When changes to a branch are pushed up, the documentation is built and available at a stable URL, a la ReadTheDocs. I believe the first part happens already via the `Web and docs` job in GitHub Actions, so I _think_ it would mostly be a matter of deploying (and later cleaning up?) the built docs. https://pandas.pydata.org/docs/dev/ shows the latest on `master`, so maybe the URL structure is something like:

```
https://pandas.pydata.org/docs/preview/<user>/<branch>/
```

Bonus: Adding a commit status that links to the generated docs for that branch, [as Netlify does](https://www.netlify.com/blog/2016/07/20/introducing-deploy-previews-in-netlify/).

#### API breaking implications

n/a

#### Describe alternatives you've considered

In one pull request, I:

1. Generated the page
2. Opened it in my browser
3. Exported it as a PDF
4. Uploaded it to Google Drive
5. Changed the sharing settings
6. Pasted the link in the comment

This solved the clickability/searchability problems, but is even more work than taking a screenshot.

#### Additional context

none
