Add deployment mode structure for drasi-lib and Drasi Server

[Copilot]

Drasi will support three deployment modes: embedded library (drasi-lib), standalone server, and Kubernetes platform. Current docs only cover platform deployment, creating confusion when new modes launch.

Changes

New Content

• /concepts/deployment-modes/ - Comparison table and decision guide for the three deployment modes
• /how-to-guides/installation/drasi-lib/ - Placeholder with API examples and feature overview
• /how-to-guides/installation/drasi-server/ - Placeholder with installation patterns and configuration examples

Updated Content

• Home page: Added deployment mode preview section with three cards
• Installation index: Restructured to present mode selection upfront
• Getting Started: Added notice specifying platform-only scope
• Concepts pages: Added deployment flexibility section and navigation card

Styling

• CSS for .deployment-modes-preview section following existing .learning-paths-preview pattern
• Responsive breakpoints for mobile

Preview

# Deployment mode comparison
| Feature         | drasi-lib   | Server      | Platform    |
|-----------------|-------------|-------------|-------------|
| Deployment      | In-process  | Standalone  | Kubernetes  |
| Management      | Code        | CLI/API     | kubectl     |
| Scaling         | Single      | Vertical    | Horizontal  |

Placeholders clearly marked as "Coming Soon" with links to GitHub/Discord for updates.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add placeholder pages for Drasi Server and drasi-lib</issue_title>
<issue_description>As part of a new Drasi release, we will be adding support for two new ways to run Drasi:

1. as a rust crate that can be used embedded in applications
2. as a stand alone server running in a single process.

This means, users will have the ability to run Drasi:

1. as a library (rust crate)
2. as a stand alone server/process
3. as a platform running on kubernetes.

Currently the documents only talk about drasi running as a platform.

These different drasi footprints are installedd, configured, and used in very different ways.

We need a content structure in place that will elegantly and clearly allow us to present these new options to the user without everything getting confusing and overwheming for the reader.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add placeholder pages for Drasi Server and drasi-lib #180

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