Docs: add hello bar in docs part

[SisyphusZheng]

Why was this PR initiated?

Currently, there is too little documentation on Fresh v2, and users often need to refer to the latest News of Fresh v2. Therefore, it may be meaningful to also place the latest News in the Docs section, so that users can realize it is the most important reference at present.

What does this PR do?

It separates the HelloBar and the Sidebar in Slug.tsx into independent components and islands, adds the HelloBar at the top of the docs section, and enables the Sidebar to rebound according to height during the dropdown process.

How was the testing conducted?

Through Deno task ok and local environment style checks.

Preview