Use GitHub Pages site origin for setting up SSG configs

[AndrewLester]

From what I could tell, two (SvelteKit and Gatsby) of the currently supported static site generators have a configuration option for the URL of the deployed site. This allows for code to access the site URL during prerendering, which is useful for social sharing, meta tags, JSON-LD, and more. Since configure-pages does have knowledge of this URL, I think it makes sense to pass it to the SSG configurations.

Let me know if this was already discussed and decided against. Also, the other generators might support a similar config option, and I would be happy to add them to this PR.

I got the name for Gatsby's config option from the docs. It's possible that users would want the pathPrefix also attached to the siteUrl, which is certainly possible. I didn't include it because it is accessible with library functions and in the previous tests with a siteURL (this was the capitalization before), it didn't include the pathPrefix.

[JamesMGreene]

Seems reasonable and makes sense.

In Gatsby, the siteMetadata.siteUrl doesn't appear to be a standard configuration, but it does appear to be a pretty common convention in plugins and such, so I'm OK with merging that bit as well. 👍🏻

Thank you! 💝

[JamesMGreene]

Apologies that this one took a little while to get merged, @AndrewLester. 😅 Thanks again for the contribution.

This is now available in the latest release (v2.1.0). 🚀

[AndrewLester]

No worries, thanks for reviewing and merging this!