Automate deploying the website

[alamb]

Right now, this site has a manual deployment process -- see https://github.com/apache/datafusion-site/blob/main/README.md#publish-site

It would be much better / less error prone to have an automated deployment mechanism so that when changes were committed to main, something automatically published updates to https://datafusion.apache.org/blog/

The actual content of the site is whatever gets pushed to the asf-site branch

Ideally the workflow would use github actions

The arrow blog has an automated workflow for jekyll that we could take a friendly look at:
https://github.com/apache/arrow-site/blob/main/.github/workflows/deploy.yml

We have had success using the https://github.com/peaceiris/actions-gh-pages action to automate publishing static content into another branch for our docs. It's built for GitHub Pages, but setting publish_branch to asf_site and publish_dir to _site might be enough to make it work for this. Here's our workflow if you want to take a look.

Originally posted by @phillipleblanc in #11 (comment)

Note however that ASF policy doesn't let us use third-party plugins (only github provided or otherwise whitelisted ones)