Add section headers to the navigation in the left sidebar

[anna-geller]

We want to add section headers that will serve as categories for the documentation to make finding the right information easier. Instead of further nesting the docs, the headers will make it easier to highlight the right sections of the docs for any stage of the adoption (getting started, building with kestra, managing kestra, scaling, etc...)

Figma: https://www.figma.com/design/g8Di3M1gbajOjOqp0ZrzXO/02_Kestra.io?node-id=12960-13588&t=Ue3jxy4m9dwBEuud-4

To determine which section will be shown under which category, we can extend this part https://github.com/kestra-io/docs/blob/9aa471873e7d369aa9ff55051f7035abffe73d84/nuxt.config.ts#L131-L136

with the specification of sections:

runtimeConfig: {
    public: {
        siteUrl: process.env.NUXT_PUBLIC_SITE_URL || 'https://kestra.io',
        apiUrl: process.env.NUXT_PUBLIC_API_URL || DEFAULT_KESTRA_API_URL,
        docs: {
            sections: {
                "Getting Started with Kestra": [
                    "Quickstart",
                    "Tutorial",
                    "Architecture",
                    "Installation Guide",
                    "User Interface",
                    "Video Tutorials"
                ],
                "Build with Kestra": [
                    "Concepts",
                    "Workflow Components",
                    "Expressions",
                    "Version Control & CI/CD",
                    "Plugin Developer Guide",
                    "How-to Guides"
                ],
                "Scale Kestra": [
                    "Enterprise Edition",
                    "Task Runners",
                    "Worker Groups",
                    "Best Practices"
                ],
                "Manage Kestra": [
                    "Administrator Guide",
                    "Configuration Guide",
                    "Migration Guide",
                    "Terraform Provider",
                    "API Reference"
                ]
            }
        }
    }
}

(credits to @wrussell1999 for all the work below, I'm only opening an issue for tracking)

[anna-geller]

[anna-geller]

already done