docs(observability): add Future AGI as a supported OTLP backend

[SuhaniNagpal7]

Summary

Adds Future AGI as a documented OTLP destination for Agent Stack telemetry, alongside the existing Phoenix and Langfuse options. Applied identically to both `docs/development` and `docs/stable`.

• Diagram — adds a `FutureAGI` node to the OpenTelemetry destinations mermaid graph, with its own styling class (matches the per-destination classes already used for `langfuse` and `custom`).
• Collector config — documents an `otlphttp/futureagi` exporter pointed at `https://api.futureagi.com/tracer/v1/traces\` with `X-API-KEY` / `X-SECRET-KEY` headers. Casing matches the traceAI repo exactly.
• Pattern parity with Langfuse — uses literal placeholders (`<YOUR_FI_API_KEY>` / `<YOUR_FI_SECRET_KEY>`) in the YAML so the start command stays a plain `agentstack platform start -f config.yaml`, and includes `filter/phoenix` in the pipeline so only OpenInference instrumentation spans get exported.
• Drive-by fix — renames the Langfuse `` placeholder to `<YOUR_AUTH_STRING>` to resolve a `mint dev` MDX compile error (hyphenated lowercase `` was being parsed as a JSX custom-element tag even inside the fenced `yaml` block).

Context

Supersedes #2621 (closed) — addresses the inline review comments from gemini-code-assist (literal placeholders, `filter/phoenix` in the pipeline, simplified start command) and rebases cleanly onto current `main`.

Verification

• Validated the YAML block from the docs with `yaml.safe_load` — parses, exporter is correctly wired into the traces pipeline.
• Sent a real OTLP/HTTP span against `https://api.futureagi.com/tracer/v1/traces\` using the exact endpoint and header names from the YAML — Future AGI's ingest accepted it (no 401/403, `force_flush` succeeded).
• Confirmed `filter/phoenix` is defined in `helm/templates/collector/config.yaml` and is the same processor already used by Phoenix and Langfuse pipelines.

Test plan

• `mint dev` renders both pages without MDX compile errors
• Mermaid diagram shows Future AGI alongside Langfuse / Custom with the new styling
• Configuring `config.yaml` per the doc + `agentstack platform start -f config.yaml` exports traces to Future AGI

[gemini-code-assist]

Code Review

This pull request adds Future AGI as an observability destination in the documentation for both development and stable environments. The changes include updating Mermaid diagrams with distinct styling for backends, clarifying Langfuse configuration placeholders, and providing a comprehensive setup guide for Future AGI. Review feedback suggests clarifying that Future AGI follows a similar configuration pattern to Langfuse and removing trailing colons from step titles to ensure consistency.

[gemini-code-assist]

The phrase "through the same collector config used for Langfuse" might be interpreted as using the exact same configuration file or settings. It's clearer to state that it uses a similar configuration pattern.

[Future AGI](https://futureagi.com) is an open-source e2e agent engineering and optimization platform that helps you ship self-improving AI agents. Its tracer endpoint accepts OpenTelemetry OTLP/HTTP, so you can route Agent Stack traces to it using a similar collector configuration pattern as Langfuse.

[gemini-code-assist]

The colon at the end of the step title is inconsistent with other step titles in this file (e.g., lines 125, 136, 141, 149, 162, 207) and is generally unnecessary for a title.

<Step title="Create a configuration file (config.yaml)">

[gemini-code-assist]

The phrase "through the same collector config used for Langfuse" might be interpreted as using the exact same configuration file or settings. It's clearer to state that it uses a similar configuration pattern.

[Future AGI](https://futureagi.com) is an open-source e2e agent engineering and optimization platform that helps you ship self-improving AI agents. Its tracer endpoint accepts OpenTelemetry OTLP/HTTP, so you can route Agent Stack traces to it using a similar collector configuration pattern as Langfuse.

[gemini-code-assist]

The colon at the end of the step title is inconsistent with other step titles in this file (e.g., lines 125, 136, 141, 149, 162, 207) and is generally unnecessary for a title.

<Step title="Create a configuration file (config.yaml)">