Unstructured SaaS API subscription guide (#2341)
To test:
> cd docs && make html
Sections:
- New User sign-up: (i) registration form, (ii) payment processing, and
(iii) use API key & URL
- API Account maintenance: (i) update billing, (ii) opt-in email, (iii)
rotate API key, and (iv) cancel plan
- Get Supports
