Add cross-references to Registry Server from existing documentation

[danbarr]

Overview

Now that the Registry Server documentation exists (PR #348), we need to add inbound references from existing ToolHive documentation so users discover it at relevant points in their journey.

The Registry Server is a fourth component of the ToolHive platform that provides standards-compliant registry hosting for teams and enterprises. Users learning about registries or setting up custom registries should be aware of this option.

High Priority - User Discovery Points

1. Main ToolHive Introduction

File: docs/toolhive/index.mdx
Location: After the three modes section (lines 46-60)
Add: Fourth bullet point introducing the Registry Server

Example:

- [**ToolHive Registry Server**](./guides-registry/index.mdx): A standards-compliant 
  registry hosting solution for teams and enterprises to discover and access MCP servers. 
  It aggregates servers from multiple sources including Kubernetes clusters, Git 
  repositories, and upstream registries with OAuth authentication and automatic sync.

2. UI Registry Guide

File: docs/toolhive/guides-ui/registry.mdx
Location: In the "Registry settings" section around line 55-72
Add: Brief mention of Registry Server as enterprise hosting option

Example note in the Remote Registry section:

:::tip[Enterprise registry hosting]

For production environments, consider using the [ToolHive Registry Server](../guides-registry/index.mdx) 
which provides enterprise-ready registry hosting with authentication, multiple data sources, 
Kubernetes discovery, and automatic synchronization.

:::

3. CLI Registry Guide

File: docs/toolhive/guides-cli/registry.mdx
Location: In the "Set a remote registry URL" subsection around line 145-156
Add: Similar tip about Registry Server as in UI guide

4. Custom Registry Tutorial (Two Locations)

File: docs/toolhive/tutorials/custom-registry.mdx

Location A: "Why create a custom registry?" section around line 16-24
Add: Mention Registry Server as alternative to file-based approach

Example:

For enterprise deployments, you can also use the [ToolHive Registry Server](../guides-registry/index.mdx) 
to host and serve your custom registry with authentication, multiple data sources, and automatic 
synchronization. This tutorial focuses on creating the registry data file itself.

Location B: "Production considerations" section around line 290-306
Add: Link to Registry Server as recommended production deployment

Example addition to the production bullet points:

- **Use the ToolHive Registry Server** for production deployments with built-in authentication, 
  multiple data sources, and Kubernetes integration (see [Registry Server documentation](../guides-registry/index.mdx))

Medium Priority - Context and Integration

5. Registry Criteria Concept

File: docs/toolhive/concepts/registry-criteria.mdx
Location: At the top around line 10-26
Add: Clarification box distinguishing registry data from Registry Server

Example:

:::note[Registry data vs Registry Server]

This page describes criteria for MCP servers in the built-in registry **data** shipped with 
ToolHive CLI and UI. For information about hosting and serving registries in production 
environments, see the [ToolHive Registry Server](../guides-registry/index.mdx) documentation.

:::

6. Kubernetes Operator Introduction

File: docs/toolhive/guides-k8s/index.mdx
Location: Introduction section around line 10-16
Add: Mention integration with Registry Server for automatic discovery

Example addition after line 16:

The operator integrates with the [ToolHive Registry Server](../guides-registry/index.mdx) 
to automatically expose deployed MCP servers through a discoverable registry, making it 
easy for users to find and connect to cluster-hosted servers.

Optional - Architecture

7. Architecture Overview

File: docs/toolhive/index.mdx
Location: "Architecture overview" section (lines 79-95)
Consideration: Could add Registry Server to the architecture diagram/description as part of the broader ecosystem (lower priority)

Success Criteria

• Users exploring registries discover the Registry Server option
• Production deployment paths point to Registry Server documentation
• Clear distinction between registry data and registry hosting
• Integration points between components are documented

Related

• PR Add ToolHive Registry documentation #348 (initial Registry Server documentation)
• Issue Improve ToolHive Registry Server documentation structure and completeness #364 (Registry Server documentation improvements)