Add workflow for deploying website after updating the documentation

[madolson]

This will allow the website to automatically get updated when we update the doc repository. I setup a repository action on the website github, which allows incoming events to trigger the build and deploy action.

Some notes about security. The credential being used has write permission the website repository, so to compensate I locked down all the branches to require PRs, which should prevent any malicious attackers if they get access to the credentials. The downside is no more directly pushing to the website, which I think is an OK tradeoff.

This PR is dependent on this PR getting merged first: valkey-io/valkey-io.github.io#214.

Some other things I investigated:

1. Re-usable workflows: I wasn't able to get the deploy functionality to have permissions across repository, these seem to only work correctly within the same repository.
2. Triggering the website build from this repo. This required duplicating the build and deploy steps. This seemed like a cleaner approach.

[madolson]

We could scope down the file paths that trigger this, but I decided to leave this as is for now. The rate of change on the documentation repository isn't that high.

[zuiderkwast]

Thank you!

Very nice. It's very satisfying to see changes deployed immediately when merging a PR. It's nice to see for the contributor too.

Requiring PRs on the website repo is a good idea regardless.