[SITE] Site staging action

[Benjamin Marwell <bm...@apache.org>]

Hi everyone!

We now have a staging action, so the site gets deployed to staging
automatically.
It is a manual run, and you need to select the source branch which
should get deployed.

https://github.com/apache/shiro-site/actions/workflows/staging.yml

On the blue bar, click the "Run workflow" dropdown and select the
source branch and click the "Run workflow" button. You can see this
bot in action by looking at the commits:
https://github.com/apache/shiro-site/commits/asf-staging

This way we can test changes (like announcements) just before or even
with the vote.

- Ben