Setup automatic deployment of the website

[mchurichi]

The spiffe.io website, even when it is a static website, it is generated using templates and scripts. There are several places in the documentation and the downloads page where we refer to the latest version of spire using its name (v0.12.3, v1.0, etc), so in order to do that, there are scripts in place that get the latest spire version tag name using the GitHub API from the spire repository and injects them into the website/documentation.

This build process is something we (mostly @sanderson042 and myself) have been manually running on Netlify's UI every time there’s a new spire version out.

We want to automate this process/continuous deployment, probably using something like GitHub Actions for Netlify