chore(layout): move slack button to header, ask ai button to bottom right

[stdrc]

Description

As title. Resolve what mentioned here: https://github.com/risingwavelabs/risingwave-docs/pull/2582#pullrequestreview-2285255657.

Currently we cannot achieve the same style of custom button as Tinybird and Neon, because RunLLM only supports placing the fix-styled button at one of the for corners but doesn't support customizing the button.

Before:

After:

Related code PR

Related doc issue

Resolve

Rendered preview

Checklist

• [x] I have checked the doc site preview, and the updated parts look good.
• [x] I have acquired the approval from the owner (and optionally the reviewers) of the code PR and at least one tech writer (emile-00, hengm3467, & WanYixian).

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