What kind of change does this PR introduce?

Documentation improvement

What is the current behavior?

The theming guide does not mention prebuilt themes at all. Developers who use a prebuilt theme have no way to know from the docs which themes use the M2 design system and which use M3. This causes confusion when components look different from the Material Design documentation site (which uses M3).

Additionally, the getting-started guide links to theming#pre-built-themes which is a broken anchor.

Closes #30565

What is the new behavior?

Adds a "Prebuilt Themes" section to the theming guide that:

• Lists all eight prebuilt themes in a table
• Identifies which themes use M3 and which use M2
• Shows the light/dark mode and color palettes for each theme
• Notes that M2 themes are provided for backwards compatibility
• Includes instructions for adding a prebuilt theme via angular.json
• Links to the source code for the prebuilt theme definitions

Also fixes the broken anchor in the getting-started guide to point to the new section.

Additional context

The prebuilt theme source files confirm:

• M3 themes (use system.theme): azure-blue, rose-red, cyan-orange, magenta-violet
• M2 themes (use m2.define-light-theme/m2.define-dark-theme): indigo-pink, deeppurple-amber, pink-bluegrey, purple-green