Migrated Grasshopper docs: missing screenshots, pages and content decisions

[daniel-locatelli]

Companion to #21 (migration) and #22 (text refresh). The text-level refresh in #22 was done from the components' metadata and code; what remains needs Rhino/Grasshopper sessions or maintainer decisions.

Screenshots needed (current toolbar/components):

• Toolbar overview on the tutorials landing page (gh_toolbar.png is pre-migration)
• Installation pages: PackageManager + resulting toolbar (gh_installation*.JPG are pre-migration)
• features.md: BTLx From Geometry, BTLx From Parameters, BTLxMachiningLimits component shots
• beams.md: Beam From Box, Beams From Mesh Edges
• fabrication.md: BeamStock, NestBeams
• utils.md: BakePlatesWithBoxMap
• show.md: ShowElementsByType, ShowElementsByCategory, Show element Reference Side (existing show images predate the renames)
• joint_rules.md: verify the rule-component screenshots still match the current component UIs

Missing pages (component families with no docs at all):

• Plates: 5 creation components + 4 plate joint rules
• Panels: 4 creation components + 4 panel joint rules
• Fasteners: Plate Fastener, Fastener Beam Interface (both say "Under maintenance!" — document or hide?)

Decisions:

• attributes.md documents components deleted in 187b7ea — reintroduce the workflow or drop the page? (Refresh migrated Grasshopper docs against the current component set #22 adds a warning banner meanwhile)
• design.md documents Surface Model / Surface Model Options, which are not exposed as components here — coming back, or drop the page? (Refresh migrated Grasshopper docs against the current component set #22 adds a warning banner meanwhile)