Summary

Fixes the embedded sitemap template alternate-link block to use .AllTranslations for translated pages.

What changed

• Updated the sitemap template to range .AllTranslations inside the existing .IsTranslated guard.
• Added a regression test covering a translated page where the current language link should be emitted before the translated language link.

Why

The previous template ranged .Translations and then manually appended the current page. That produced the right set of links, but it duplicated the behavior that .AllTranslations already provides and made the current-language entry come from a separate code path. Keeping .IsTranslated preserves the existing behavior for single-language sites.

Testing

• Before the template change, go test ./hugolib -run TestSitemapMultilingualAlternateLinksUseAllTranslations -count=1 failed because the current language link was emitted after the translated language link.
• go test ./hugolib -run TestSitemapMultilingualAlternateLinksUseAllTranslations -count=1
• go test ./hugolib -run 'TestSitemap' -count=1
• git diff --check
• PATH="$(go env GOPATH)/bin:$PATH" ./check.sh ./hugolib
• PATH="$(go env GOPATH)/bin:$PATH" ./check.sh ./tpl/tplimpl

I also ran PATH="$(go env GOPATH)/bin:$PATH" ./check.sh; it passed gofmt and staticcheck, then failed in hugolib/pagesfromdata because local external converters were missing (asciidoctor; with warnings for missing pandoc and rst2html).

Fixes #14912

Note: I used Codex to help inspect the issue, prepare the narrow fix, and run local verification; I reviewed the final diff and the listed command results before opening this PR.