docs(blocks): Add AI/ML API integration guide and update LLM headers (#10402)

### Summary
Added a new documentation page and images for integrating AI/ML API with
AutoGPT, including step-by-step instructions. Updated LLM block to send
additional headers for requests to aimlapi.com. Improved provider
listing in index.md and added the new guide to mkdocs navigation. Builds
on and extends the integration work from
https://github.com/Significant-Gravitas/AutoGPT/pull/9996


### Changes 🏗️

This PR introduces official support and documentation for using **AI/ML
API** with the **AutoGPT platform**:

* 📄 **Added a new documentation page** `platform/aimlapi.md` with a
detailed step-by-step integration guide.
* 🖼️ **Added 12+ reference images** to `docs/content/imgs/aimlapi/` for
clear visual walkthrough.
* 🧠 **Updated the LLM block** (`llm.py`) to send extra headers
(`X-Project`, `X-Title`, `Referer`) in requests to `aimlapi.com` for
analytics and source attribution.
* 📚 **Improved provider listing** in `index.md` — added section about
AI/ML API models and benefits.
* 🧭 **Added the new guide to the mkdocs navigation** via `mkdocs.yml`.

---

### Checklist 📋

#### For code changes:

* [x] I have clearly listed my changes in the PR description
* [x] I have made a test plan
* [x] I have tested my changes according to the test plan:

  * [x] Successfully authenticated against `api.aimlapi.com`
  * [x] Verified requests use correct headers
* [x] Confirmed `AI Text Generator` block returns completions for all
supported models
* [x] End-to-end tested: created, saved, and ran agent with AI/ML API
successfully
  * [x] Verified outputs render correctly in the Output panel


No breaking changes introduced. Let me know if you'd like this guide
cross-referenced from other onboarding pages. ✅

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Co-authored-by: Nicholas Tindle <[email protected]>