Docs/catch up 2024.2 to 2026.2

[nahumtimerman]

Brings documentation up to date — we were frozen at 2024.1 GA. Removes old versioned-docs snapshots (single "current" version strategy) and adds all missing feature documentation.

Pages to review

Release Notes section:

• /release-notes/whats-new — Rewritten, covers
  2024.2 through 2026.2 (newest first)
• /release-notes/behavior-change — Added
  2026.1 (Node x64, Erlang 26, MongoDB 8) and 2026.2 (new defaults)
• /release-notes/supported-versions — Added
  2025.1 & 2026.1 rows, grayed out expired versions

New feature pages:

• /admin/cloudshell-manage-dashboard/maintenance-window — Maintenance Window by Domain
• /portal/sandboxes/sandbox-workspace/export-diagram-png — Export Diagram as PNG

API Guide (6 new features):

• Start_Time parameter, CancelPending, UndeployApps, RemoveEntities, DomainSettings, AttributeChangedEvent

Portal/Sandbox Workspace:

• Sticky notes, copy resource text, state filter, save confirmation
• Assembly Lab: collapse panel, route drawing, L1 solving, exclusive resources

Admin/Config:

• New config keys in general.md: QS_ env override, Unicode, AI Chat URL
• Execution Server prerequisites updates (Node x64, Erlang 26, MongoDB 8)

Structural changes

• Deleted versioned_docs/ and versioned_sidebars/ (~140K lines removed)
• versions.json → empty array (no more version dropdown)
• Category label: "CloudShell Release Notes" (was "CloudShell
  2024.1 Release Notes")

Not in scope

• API doc static files (static/api-docs/) still at
  2024.1 — will update separately
• "Added in CloudShell X" historical annotations left intentionally