6.x: Improve documentation navigation and homepage

[josbeir]

The sidebar mixes framework concepts with development tasks, making important guides difficult to find. The English homepage also duplicates lengthy tutorials and feature descriptions.

This change makes the documentation easier to navigate:

• Reorganize English and Japanese sidebars into clearer groups with shorter labels and shallower nesting.
• Preserve existing links and add the missing Locking page.
• Replace the English homepage with a welcoming hub featuring starting points, a compact quick start, guide links, and community resources.
• Add reusable Vue components and responsive homepage styling for light and dark modes.
• Preserve the sidebar, search, and version banner; correct heading-anchor positioning.
• Replace Quickstart's homepage include with standalone help links.

Why restructure the sidebar?

The previous sidebar was cluttered and mixed framework concepts, development tasks, and miscellaneous topics. Important guides were hidden behind broad categories: configuration lived under “Development,” deployment under “Additional Topics,” and authentication under “Plugins.” Clearer groups, shorter labels, and shallower nesting make these topics easier to discover without already knowing where to look.

Navigation examples

Topic	Before	After
Configuration	Development → Configuration	Start Here → Configuration
Deployment	Additional Topics → Deployment	Testing & Deploy → Deployment
Authentication	Plugins → Core Plugins → Authentication	Security → Authentication ↗
Migrations	Plugins → Core Plugins → Migrations	Database → Migrations ↗
CMS tutorial	Getting Started → Tutorials & Examples → CMS Tutorial → Installation	Start Here → CMS Tutorial → Installation
Form protection	Controllers → Components → Form Protection	Security → Form Protection
Encryption and hashing	Core Libraries → Security	Security → Encryption & Hashing
REST APIs	Development → REST	HTTP → REST APIs

[LordSimal]

I like it 👍🏻