Gateway docs: reword invite-only Quick Start once self-serve is live

[swarna1101]

docs/versions/v1.3.2/01_quick_start.md:18 in getoptimum/optimum-gateway says:

Access is invite-only. You cannot self-register. The Optimum team must onboard you first…

This contradicts self-serve signup. Change it only once self-serve is live in prod: the signup, self-serve role and Signal flags are currently off in prod.

Tasks

• Remove the invite-only callout.
• Lead with the self-serve path: Console, then a join key (OPT_JOIN_KEY). Link to Gateway Self-Enrollment.
• Keep the ogw_ API-key flow as the partner path.

Done when

The Quick Start matches prod Console access.

[linear-code]

OPT-1120