poc(api): add date-based per-operation API versioning

[nogates]

Summary

Draft POC for date-based per-operation API versioning. Split from #38028, which now contains only the API changelog page.

• Operations flagged x-datadog-api-versioned render a per-operation version selector with available dates, lifecycle tags, and a Follow global action.
• A Global API version control in the sidebar sets the baseline date for versioned operations unless an operation has an override.
• Request and response schema panes render one variant per version with New in and Removed in change markers.
• Large oneOf pruning is scoped to date-versioned schemas so generated examples remain below the GitHub file-size limit without affecting unrelated schemas.

This is a design POC for reviewer walkthrough, not intended to merge as-is.

Test plan

• Visit /api/latest/dashboards/create-a-new-dashboard/ and confirm the per-operation version selector lists 2026-04-01, 2026-06-01, and 2026-07-01 with the correct lifecycle tags.
• Confirm the sidebar Global API version selector changes the baseline version for operations following it.
• Confirm per-operation overrides persist independently and Follow global clears an override.
• Confirm New in and Removed in markers render on the version pane where each schema change was introduced.
• Confirm the sidebar version selector aligns with the search box and navigation tree.