Remove localized case studies content

[Kyle-Law]

Follow-up to kubernetes#57043, which removed the English case studies section.

Extends the same removal to every other locale that still had translated case studies content:

• bn (194 files)
• zh-cn (212 files)
• vi (190 files)
• ja (14 files)
• ko (6 files)
• it (4 files)
• pt-br (3 files)

For each locale:

• Deletes content/<lang>/case-studies/ entirely.
• Drops the {{< blocks/case-studies >}} homepage shortcode call from content/<lang>/_index.html, matching how Remove case studies kubernetes/website#57043 trimmed content/en/_index.html.
• content/zh-cn/community/_index.html was the only locale page linking directly into the section (/zh-cn/case-studies); repointed it to CNCF's case studies page, matching the fix already applied to the English community page in Remove case studies kubernetes/website#57043.

Not included (left for a separate follow-up so this stays focused on content removal): layouts/shortcodes/blocks/case-studies.html, layouts/case-studies/list.html, the case-studies.css wiring in layouts/partials/css.html, and the remaining per-company static/css/style_*.css files. These become fully dead code once every locale's case studies are gone, but that's a template/CSS cleanup, not a content change.

Note on process: localized content is normally under each language SIG's own review - this PR only handles the mechanical removal + the one broken link it would otherwise create. Worth a heads-up to the relevant l10n subprojects before merging, same as was flagged for the English removal in kubernetes#57043.

[Kyle-Law]

Closing this. On reflection this was premature. it assumes a decision ("remove case studies for every locale") that hasn't actually been made.

The only two maintainer-side signals on this are:

• katcosgrove's own review reply on Remove case studies kubernetes/website#57043: "We can't remove the CSS. The case studies still exist in other localizations." (Remove case studies kubernetes/website#57043 (comment))
• lmktfy's comment asking to defer unholding the English-only removal until after the 1.37 release and to "tip off localization subproject(s) about the time line" first (Remove case studies kubernetes/website#57043 (comment)) — i.e. notify, then let those subprojects decide, not remove unilaterally.

That heads-up hasn't happened yet even for the English change, and each of these locales (bn, it, ja, ko, pt-br, vi, zh-cn) has its own OWNERS/approvers who'd need to weigh in on removing their translated content. Closing rather than leaving this open as a live proposal ahead of that process.