Cortex-R5/GNU: Migrate to Markdown documentation

This replaces readme_threadx.txt with Markdown documentation including
TCM/DDR stack placement policy and ECC memory handling.

Stack Placement Policy:
- System/ISR stacks in ATCM for deterministic interrupt response
- Thread stacks in DDR for abundant capacity
- Verified context switch code is memory-region agnostic

ECC Implementation:
- Complete ATCM/BTCM initialization sequence with proper barriers
- Correct DFSR fault status encoding (FS[5:0])
- Implementation-defined ACTLR bit warnings with vendor mappings
- Three fault handling strategies (Panic/Reset, Isolate, Redundant)
- ISO 26262 (ASIL-A to ASIL-D) and IEC 62304 (Class A-C) requirements
- Memory scrubbing for high-reliability systems

Author: jserv