CI: use the shared check-structure.php from doc-base

[lacatoire]

Replaces the local copy of check-structure.php with the shared one from php/doc-base (merged in php/doc-base#342), so that every translation runs the same version.

The workflow now checks out the translation, en and doc-base side by side, the layout the doc-base scripts expect and the one the build workflow already uses.

[KentarouTakeda]

Thanks. The points in #412 (comment) still stand.

Two of them are now in doc-base, so we can't fix them here. A divergence that is only a difference in nesting depth still prints the same signature on both sides. The translator-credits exemption still doesn't match ours, so preface.xml and bookinfo.xml can never pass.

Closing. Happy to revisit once those land upstream and the check looks at changed hunks rather than whole files.