Big docs tidy up in preparation for launch (#364)

## Motivation and Context
Docs currently don't have very clear organisation, and don't explain
core flows.

This fixes #89 (although we still might want to push these to
`https://modelcontextprotocol.io/docs/`), based on the plan there.

One minor change from that plan is that I merged tutorials and guides,
because they ended up basically being duplicates which I thought would
be annoying for us to keep in sync. Additionally in the Diátaxis
framework, tutorials and guides are separated by whether they are
focused on acquisition of skills (e.g. in a learning environment) or
application of skills (e.g. in a work environment). I expect the
registry to be something that will be used occasionally in a work
environment, rather than a fundamental 'learnable' skill so I think it's
reasonable not to have tutorials and fold these into guides.

## How Has This Been Tested?
- Read through all these updated docs
- Unit and integration tests (that previously used examples.md and use
the schema) working
- Ran `npx markdown-link-check .` to check all the links work

## Breaking Changes
Links to old docs will break, but I don't think there are any. All
internal links have been checked and updated.

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

docs/README.md

@@ -1,18 +1,16 @@
-# Official Registry Documentation
+# MCP Registry Documentation
 
-This extends [the main README](../README.md).
+The MCP registry provides MCP clients with a list of MCP servers, like an app store for MCP servers.
 
-## Project Documentation
+## I want to...
 
-- [`faq.md`](./faq.md) - Frequently asked questions about the MCP Registry
-- [`design_principles.md`](./design_principles.md) - Core constraints and principles guiding the registry design
-- [`roadmap.md`](./roadmap.md) - High-level roadmap for the MCP Registry development
-- [`MCP Developers Summit 2025 - Registry Talk Slides.pdf`](./MCP%20Developers%20Summit%202025%20-%20Registry%20Talk%20Slides.pdf) - Slides from a talk given at the MCP Developers Summit on May 23, 2025, with an up-to-date vision of how we are thinking about the official registry.
+- **📤 Publish my MCP server** → [Publishing Guide](guides/publishing/publish-server.md)
+- **📥 Consume registry data** → [API Usage Guide](guides/consuming/use-rest-api.md)
+- **🔌 Understand the registry's purpose** → [Ecosystem vision](explanations/ecosystem-vision.md)
+- **📋 Look up specific information** → [server.json spec](reference/server-json/generic-server-json.md) | [API spec](reference/api/generic-registry-api.md) | [CLI reference](reference/cli/commands.md)
 
-## API & Technical Specifications
+## Documentation Index
 
-- [`openapi.yaml`](./server-registry-api/openapi.yaml) - OpenAPI specification for the official registry API
-- [`api_examples.md`](./api_examples.md) - Examples of what data will actually look like coming from the official registry API
-- [`architecture.md`](./architecture.md) - Technical architecture, deployment strategies, and data flows
-- [`server.json` README](./server-json/README.md) - description of the `server.json` purpose and schema
-- [`new_package_registry.md`](./new_package_registry.md) - steps to add a new package registry for local server packages
+- 🛠️ [How-To Guides: Task-focused instructions](./guides/)
+- 💡 [Explanations: Understanding-oriented content](./explanations/)
+- 📖 [Reference: Technical specifications and lookup material](./reference/)