[Docs] Capture federation subplan pushdown

[manfredcml]

What does this PR do?

Federation now pushes a whole single-connection subplan into its source where Arris can write that source's SQL dialect, but the cross-source page still described pushdown as per-table projections and filters. Also, Canvas is no longer new, so the docs index stops announcing it as a release headline.

Changes:

• Pushdown copy: rewrite the "Pushes down" and "Executes" steps, and name the seven sources that get subplan pushdown (PostgreSQL, Redshift, MySQL, MariaDB, SQLite, DuckDB, BigQuery)
• Execution plan: explain that a pushed-down subplan collapses to a single Scan: <connection> node, while a plain table read names the table
• Limitations: retune "Data volume" and "Engine-specific features" for pushdown, since DataFusion still plans the query even when the work runs at the source
• Canvas: drop the "New in this release" paragraph and the "New:" card prefix on the docs index
• Remove stale Cassandra resources

Checklist

• I opened an issue first for non-trivial changes, or this is a small fix.
• Tests added/updated for my change.
• I license my contribution under the MIT License, per CONTRIBUTING.md. I confirm this is my original work, or that I have the right to submit it.