docs: Edge Proxy adoption decision guide (complement to operational guide)

## Gap

The [Edge Proxy operational guide](https://docs.flagsmith.com/advanced-use/edge-proxy) covers how to deploy and configure the proxy. It does not cover **when** a customer should adopt it. Customers consistently ask this during evaluation and during scaling decisions, and the answer is currently improvised by sales engineering and CS.

## Proposed scope

A short decision guide as a sibling to the operational page.

**Primary trigger**: performance / latency concerns, typically driven by geographic distribution of users.
**Secondary trigger**: data residency needs that core API regions cannot satisfy.

**Not a primary trigger**: API cost or rate constraints. Edge Proxy is a last resort for cost reduction because (a) it introduces more infrastructure to manage and another failure point, (b) for self-hosted customers, moving API load elsewhere often shifts infra cost rather than reducing it, (c) if you can accept the proxy's limitations on identity/trait persistence, local evaluation often solves the same problem more simply.

**Decision flow**:
1. Do you have performance or latency concerns? If no, do not add Edge Proxy.
2. If yes, can local evaluation solve it? If yes, prefer local evaluation.
3. If no, Edge Proxy is the right tool. Operational ownership: customer operates it themselves on self-hosted; Flagsmith operates it on the customer's behalf on Private Cloud.

Closes the "when to adopt" gap that the operational guide does not cover.