Rewrite FilterLibrary docs to replace deleted FilterCache/FilterGroup references

[jschick04]

With PRs #564 (FilterLibrary), #567 (legacy deletion + rewire to FilterLibrary) merged, the user docs still describe the deleted FilterCache and FilterGroup features. They need a proper rewrite to describe the new FilterLibrary model (Saved tab + Favorites tab + Filter Sets).

Files to update

• docs/Home.md — TOC entry currently reads [Saved Filters] — Filter cache (Favorites + Recent) and Filter groups
• docs/Filtering.md — pane header descriptions reference Save Filters as Group, Manage Cached Filters, Manage Filter Groups (all renamed or replaced by FilterLibrary)
• docs/Keyboard-And-Copy.md — references to Save Filters as Group action and the New Filter Section\New Filter Group default prompt
• docs/Saved-Filters.md — entire section on filter groups and filter cache modal needs rewrite (modals are deleted)
• docs/.images/filter-groups-modal.png — screenshot of deleted modal; replace with FilterLibraryModal at Saved tab
• docs/.images/filter-cache-modal.png — screenshot of deleted modal; replace with FilterLibraryModal at Favorites tab

Current UI/API state to document

• Filter Library modal (FilterLibraryModal.razor) with 3 tabs: Saved, Favorites, Previously Used
• Filter Set terminology (was Preset, was Filter Group) — a named collection of filters that applies as a unit
• Save as Filter Set action on FilterPane header (was Save Filters as Group)
• Manage Saved Filters icon button on FilterPane (was Manage Filter Groups) → opens FilterLibrary at Saved tab
• Manage Cached Filters icon button remains, opens FilterLibrary at Favorites tab