feat: support unhashable arguments in cache key generation

[27Bslash6]

Description

Competitive testing confirms cachekit handles unhashable arguments (lists, dicts, nested structures) that crash lru_cache and cachetools. This is a genuine competitive advantage worth highlighting.

Evidence

# lru_cache — crashes
@lru_cache(maxsize=128)
def fn(data): return sum(data)
fn([1, 2, 3])  # TypeError: unhashable type: 'list'

# cachetools — crashes
@cached(cache=TTLCache(maxsize=128, ttl=300))
def fn(data): return sum(data)
fn([1, 2, 3])  # TypeError: unhashable type: 'list'

# cachekit — works
@cache(backend=None, ttl=300)
def fn(data): return sum(data)
fn([1, 2, 3])  # 6 ✓
fn({"value": 21})  # works via content-based hashing ✓
fn({"users": [{"name": "alice", "tags": ["admin"]}]})  # nested ✓

Current State

This already works via content-based hashing (Blake2b of MessagePack-serialized args). However:

1. It's not prominently documented as a feature
2. The comparison doc doesn't call it out as an advantage
3. There are no docs explaining HOW it works (content-based hashing)

Suggested Actions

1. Add "Unhashable Arguments" section to comparison.md
2. Document content-based key generation in backends/index.md
3. Add examples to getting-started.md showing dict/list args