doc : add FIM architecture documentation

Comprehensive HTML documentation illustrating the FIM (Fill-in-the-Middle)
completion system architecture, covering:

- Request flow with cache hit/miss branches
- Trigger mechanism with dual autocmds
- Local context construction (prefix/middle/suffix)
- LRU cache system with SHA-256 hashing
- Fuzzy matching over 128 typed characters
- Ring buffer context with similarity eviction
- Request payload and response fields
- Response rendering with repetition filtering
- Speculative pipeline for KV cache warm-up
- Accept modes and keybindings
- Timing breakdown
- Full configuration reference

Includes interactive demo for fuzzy matching visualization.

Assisted-by: llama.cpp:local pi

Author: ggerganov