[Content Migration] Migrate Handbook to Nuxt Content

### Epic

4867

### Description

## Objective
Migrate the `/handbook` section from 11ty to Nuxt using the Nuxt Content module.

## Tasks
- [ ] Install and configure `@nuxt/content` in the Nuxt project.
- [ ] Recreate the Handbook layout/wrapper in Vue (`pages/handbook/[...slug].vue`).
- [ ] Move the legacy markdown files from 11ty into Nuxt's `content/handbook/` directory.
- [ ] Refactor any 11ty specific shortcodes/nunjucks macros in the markdown to Nuxt Content MDC (Markdown Components).
- [ ] Remove the proxy routing rule for `/handbook/**` in `nuxt.config.ts`.
- [ ] Delete the handbook source files from the `/legacy` folder.

## Acceptance Criteria
- The `/handbook` route is entirely powered by Nuxt and Vue.
- All internal links within the handbook work.
- The 11ty codebase no longer builds the handbook.

### Acceptance Criteria

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate