fix: correct base URL for Pages deployments

[zlataovce]

Fixes broken metadata and makes the sitemap generate once again. To me, it seems like there's no way to get a deployment URL at build time, so this PR tries to guess it for previews.

Based on my observations, deployment URLs are formatted in the https://<id>.papermc-docs.pages.dev format, where <id> is a <PR head branch owner>-<branch name> identifier capped to 28 characters and special characters (/, _, .) replaced with a dash. Examples:

• https://papermc-feat-add-trial-chamb.papermc-docs.pages.dev
• https://zlataovce-fix-highlight-cont.papermc-docs.pages.dev
• https://papermc-121.papermc-docs.pages.dev

[github-actions[bot]]

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Name	Status	Preview	Last Commit
papermc-docs	✅ Ready (View Log)	Visit Preview	ffe9ac5e5889227d3ea2877271b9faacf08f94ea