docs: add market size analysis for sample pack creators

Commit notes

Market estimates: - 5-10M beat producers globally - 500k-1M active sellers across platforms - 50k-100k sample pack creators specifically - 5k-15k small labels/collectives

Platform benchmarks: - Splice: 4M+ users, ~30k creators - Beatstars: 500k+ producers - Loopmasters: ~5k creators

Year 1 target: 1% of active sellers = 5-10k users At 10k users with new model = ~€12k MRR

Added competitive positioning and acquisition channels.

Co-authored-by: armin.naimi <[email redacted]>