docs: add module documentation for dm_sw_ring

[Copilot]

README.md was a 2-line stub and docs/ did not exist. Adds full documentation following the dmgpio reference structure.

README.md

• Features list with flag names, quick-start snippet, build instructions, project structure tree, related projects

docs/README.md

• Documentation index with a quick-reference code block covering all major operations

docs/dm_sw_ring.md

• Module overview: architecture diagram, ring buffer mechanics, behavior flags table, thread-safety model

docs/api-reference.md

• All 3 public types and all 12 public functions — signatures, parameter semantics, return values, and a write-overflow behavior table

docs/examples.md

Six annotated examples:

1. Simple FIFO read/write
2. Overflow with drop_old_data
3. Blocking RTOS producer/consumer using dm_sw_ring_flags_default
4. Non-destructive peek for length-prefixed framing
5. discard / clear
6. State inspection (capacity, size, available_space, is_full, is_empty)

Quick-start snippet from README:

dm_sw_ring_t ring = dm_sw_ring_create(64, dm_sw_ring_flags_default);

uint8_t tx[] = {1, 2, 3, 4};
dm_sw_ring_write(ring, tx, sizeof(tx));

uint8_t rx[4];
dm_sw_ring_read(ring, rx, sizeof(rx));

dm_sw_ring_destroy(ring);