docs: Added page "How it works"

[Swiftaxe]

Summary

The documentation currently lacks a dedicated page explaining Serverpod's architecture. This makes it harder for new users to understand how the different parts of the framework fit together.

This PR adds a new "How Serverpod works" page that provides a concise architectural overview, covering:

• The project structure
• Code generation
• Request lifecycle
• Full-stack type safety

The focus for this page is on how full-stack type safety is achieved through generating the client project and ORM classes.

Images

Notes for reviewer

• As we will move this page into "get started", I assume there is no need for an icon in the sidebar. For now, I kept the icon, previously used for Overview.
• The page has a slightly shorter name in the menu, "How it works", while being called "How Serverpod works" on the page. This is intentional. "Serverpod" provides a better context and SEO perspective on the page, but is inferred in the menu.
• Verify it covers the essential topics, so that a new user can easily grasp how the core of ServerPod works.
• Verify the length is short enough to fit in the get started path.

Test plan

• npm run build passes (no broken links).
• Render /next locally and confirm the page reads well end-to-end.
• Confirm the sidebar lists the new page correctly.

[developerjamiu]

Document flows well top to bottom. Most of my line-level comments come from the doc templates and Viktor's feedback on the Introduction PR (#495).