[course] refine Doris 101 learning experience

[morningman]

Summary

• clarify the Doris 101 landing-page introduction and show that more courses are coming
• surface module descriptions from the lesson navigation on hover and keyboard focus
• move baseline information into the lesson header and refine active-module and flow layouts across desktop and mobile
• keep the new UI aligned with the existing semantic brand-theme tokens

Validation

• git diff --check
• targeted English Docusaurus build with the course plugin enabled

Scope

This changes the shared course UI only; no versioned or localized documentation content is modified.