Update architecture doc for saasv2

[mike-solomon]

• Replaces the existing architecture doc with an updated version reflecting the SaaS v2 platform
• Uses "Connector" terminology (instead of "agent") throughout this doc — a docs-wide rename of other pages is a separate effort
• Replaces the static PNG architecture diagram with a simplified Mermaid diagram showing the v2 service topology
• Adds new sections for Changelog, Moddy, DevCenter, and Moderne Trigrep with links to detailed docs
• Replaces superseded sections (Netflix Eureka, generic worker/recipe execution/SCM/tokens services) with v2 microservice descriptions (Organization Management, Recipe Marketplace + Worker, Changeset Services, Authorization Service, Audit Service)
• Preserves the encryption/security narrative and all setup requirements unchanged
• Updates the API gateway section to describe Apollo Router federation

This is largely based on AI documentation created by @scuba10steve

Open questions I have:

• Did the artifact storage model change at all? I just left it the same but it's not clear to me if that's accurate.
• Likewise, did the encryption model change at all? I left it as is (customer-held symmetric key, decrypt-on-use, revocable by shutting off Connector) - but unsure if that's accurate.

[scuba10steve]

We'll want to check on the Artifact Storage service as that no longer exists in the monorepo.

[mike-solomon]

I've removed the artifact storage section from the doc.

Are LST artifacts fetched from the customer artifact repos through the Connector rather than being pre-copied into Moderne's cloud storage?

Do we still store these artifacts our cloud?

[mike-solomon]

Closing this PR in favor of a SaaS v2 branch where all of the changes will come in at once: #591