Document chamfer and fillet help targets

[trailcode]

Summary

Chamfer and Fillet now have their own usage-guide sections, and the mode ? buttons open those sections. Square, rectangle, and slot Add midpoint nodes stay on the existing line-edge midpoint page. That page notes they share the control because those shapes are built from straight edges, and it records the code defaults (square/rectangle on; slot straight edges only, off).

Orthographic projection is unchanged.

New sections

• Chamfer Tool (C) — docs/usage.md, anchor #chamfer-tool-c. Selection, Chamfer Mode (Edge, Wire, Face, Shape), Chamfer dist, and project units.
• Fillet Tool (F) — docs/usage.md, anchor #fillet-tool-f. Same pattern for Fillet Mode and Fillet radius.

Other Feature Operations keeps a short pointer to both sections.

? controls

Control	Opens
Chamfer Options header ? (Mode::Shape_chamfer in get_doc_url_for_mode)	#chamfer-tool-c
Fillet Options header ? (Mode::Shape_fillet in get_doc_url_for_mode)	#fillet-tool-f
Add midpoint nodes on Add square, Add rectangle from two points, Add rectangle with center point, and Add slot	existing #line-edge-option-add-midpoint-nodes
Settings → Sketch → Nodes: Add midpoints to line edges, Add midpoints to square/rectangle, Add midpoints to slot edges	existing #line-edge-option-add-midpoint-nodes

doc_urls::k_line_edge_midpoint_nodes is unchanged. Line, multi-line, and arc midpoint help stays on that same URL.

Verification

sphinx-build -b html -W docs docs/_build succeeded. The built HTML contains chamfer-tool-c, fillet-tool-f, and line-edge-option-add-midpoint-nodes.