docs: rewrite SPONSORSHIP.md with clearer structure and comparison table

Rewrites the sponsorship page to be more direct and persuasive:
- Reframes intro as facts rather than first-person claims
- Renames 'By the Numbers' to 'Audience' and adds who the visitors are
- Expands tier descriptions with explicit bullet lists of deliverables
- Cleans up past sponsors section formatting
- Simplifies the closing call to action

Co-Authored-By: Claude <[email protected]>