Add GitHub Action for site and preview deployments

[danielledeleo]

We now self-host the Unit docs site on Azure alongside other NGINX docs. This change adds a GitHub Action to build, preview, and deploy to the new environment. Some Netlify functionality, like previews has been replicated.

Related PR: https://github.com/nginxinc/kubernetes-ingress/pull/6027

Action Documentation: https://github.com/nginxinc/docs-actions/tree/main?tab=readme-ov-file#docs-actions

[danielledeleo]

I won't be able to test this action until it is merged (🐓🥚). Merging it will not affect the live site, as deployments to production must be done by running the workflow manually.

[callahad]

Concerned about dropping the redirects / url proxying from netlify.toml without the commit message saying anything about what's happened to them.

...also a bit nervous about the record of those disappearing from this repo; knowing to redirect /download and transparently proxy /download/* seems pretty crucial to making the site actually work.

[nginx-jack]

Concerned about dropping the redirects / url proxying from netlify.toml without the commit message saying anything about what's happened to them.

...also a bit nervous about the record of those disappearing from this repo; knowing to redirect /download and transparently proxy /download/* seems pretty crucial to making the site actually work.

We can add a commit message explaining that those redirects now live in a private repository as an NGINX configuration, and add something about it to a readme as well, so if/when unit changes hosting again, we make sure we don't lose those redirects.

I'd like to get to a point, like netlify, where each documentation resource can provide it's own NGINX snippet containing it's required proxying and redirects, but we're not there yet unfortunately.

[github-actions[bot]]

✅ Deploy Preview will be available once build job completes!

Name	Link
😎 Deploy Preview	https://frontdoor-test-docs.nginx.com/previews/unit/159/

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