feat(docs): add Giscus comment system using GitHub Discussions

[zywoote3ture]

What problem were solved in this pull request?

Issue Number: #598

Add a discussion board to the documentation site using Giscus.

Features

• Powered by GitHub Discussions (no extra Issues created)
• One Discussion per page (mapped by URL path)
• Supports Markdown, replies, reactions, dark/light mode, and Chinese
• Fully static — no backend required

Changes

• Added docs/overrides/partials/content.html
• Updated mkdocs.yml to enable custom_dir: overrides

Requirements (for maintainers)

• Enable Discussions in repository settings
• Install Giscus App: https://github.com/apps/giscus
• Create a category named "Docs Feedback" (Announcements type)

[CLAassistant]

All committers have signed the CLA.

[hnwyllmm]

Looks good. Thanks.

Merged by #627