docs: Reorganize documentation on the webpage (#2651)

# What does this PR do?
Reorganizes the Llama stack webpage into more concise index pages,
introduce more of a workflow, and reduce repetition of content.

New nav structure so far based on #2637 

Further discussions in
https://github.com/meta-llama/llama-stack/discussions/2585

**Preview:**
![Screenshot 2025-07-09 at 2 31
53 PM](https://github.com/user-attachments/assets/4c1f3845-b328-4f12-9f20-3f09375007af)

You can also build a full local preview locally 

 **Feedback**
Looking for feedback on page titles and general feedback on the new
structure

**Follow up documentation**
I plan on reducing some sections and standardizing some terminology in a
follow up PR.
More discussions on that in
https://github.com/meta-llama/llama-stack/discussions/2585

docs/source/introduction/index.md

@@ -1,62 +0,0 @@
-# Why Llama Stack?
-
-Building production AI applications today requires solving multiple challenges:
-
-**Infrastructure Complexity**
-- Running large language models efficiently requires specialized infrastructure.
-- Different deployment scenarios (local development, cloud, edge) need different solutions.
-- Moving from development to production often requires significant rework.
-
-**Essential Capabilities**
-- Safety guardrails and content filtering are necessary in an enterprise setting.
-- Just model inference is not enough - Knowledge retrieval and RAG capabilities are required.
-- Nearly any application needs composable multi-step workflows.
-- Finally, without monitoring, observability and evaluation, you end up operating in the dark.
-
-**Lack of Flexibility and Choice**
-- Directly integrating with multiple providers creates tight coupling.
-- Different providers have different APIs and abstractions.
-- Changing providers requires significant code changes.
-
-
-### Our Solution: A Universal Stack
-
-```{image} ../../_static/llama-stack.png
-:alt: Llama Stack
-:width: 400px
-```
-
-Llama Stack addresses these challenges through a service-oriented, API-first approach:
-
-**Develop Anywhere, Deploy Everywhere**
-- Start locally with CPU-only setups
-- Move to GPU acceleration when needed
-- Deploy to cloud or edge without code changes
-- Same APIs and developer experience everywhere
-
-**Production-Ready Building Blocks**
-- Pre-built safety guardrails and content filtering
-- Built-in RAG and agent capabilities
-- Comprehensive evaluation toolkit
-- Full observability and monitoring
-
-**True Provider Independence**
-- Swap providers without application changes
-- Mix and match best-in-class implementations
-- Federation and fallback support
-- No vendor lock-in
-
-**Robust Ecosystem**
-- Llama Stack is already integrated with distribution partners (cloud providers, hardware vendors, and AI-focused companies).
-- Ecosystem offers tailored infrastructure, software, and services for deploying a variety of models.
-
-
-### Our Philosophy
-
-- **Service-Oriented**: REST APIs enforce clean interfaces and enable seamless transitions across different environments.
-- **Composability**: Every component is independent but works together seamlessly
-- **Production Ready**: Built for real-world applications, not just demos
-- **Turnkey Solutions**: Easy to deploy built in solutions for popular deployment scenarios
-
-
-With Llama Stack, you can focus on building your application while we handle the infrastructure complexity, essential capabilities, and provider integrations.