docs(library): add Supermodel (Sega Model 3) core documentation

[sgiannop]

Summary

• Adds docs/library/supermodel.md — full documentation page for the Supermodel libretro core (Sega Model 3 arcade emulator)
• Registers the core in mkdocs.yml under the Sega Emulation section
• Adds an entry to docs/guides/core-list.md

Core details

• Hardware: Sega Model 3 (1996–1998): Virtua Fighter 3, Scud Race, Daytona USA 2, The House of the Dead 2, etc.
• License: GPLv3
• Authors: Supermodel Team (original), twinaphex (initial libretro port), sgiannop, beudbeud
• Repo: https://github.com/sgiannop/Libretro-Supermodel

What's documented

• Background, requirements (OpenGL 3.3+ / GLES 3.0+), extensions, features matrix
• Required config files (Games.xml, Supermodel.ini)
• Directories (system, save, state)
• Geometry and timing (57.53 Hz, 496×384, 44100 Hz)
• All 15 core options across Video / Input / Audio / Advanced categories
• Joypad and Lightgun input tables
• Rumble / force feedback notes
• Compatibility notes and external links