Optimize exported Markdown style, add H2

[zhoukuncheng]

Avoid poor Markdown rendering, because GPT often outputs content with an H3 title, which looks awkward when following a user message.

I recently began taking extensive notes from ChatHub to Obsidian, so this PR will benefit people who rely on ChatHub for note-taking.

最近靠 ChatHub 总结笔记，导出的 md 格式，author 是正文，而 GPT 输出包含了多级标题，就很奇怪。
修改后： author 作为 H2

before:

after:

[wong2]

感觉这样改也不是很合理，user的输入和bot的输出应该是同级的

[zhoukuncheng]

如果平级的话，得考虑 bot 长串文字的处理，可能加辅助文本会好一点

[zhoukuncheng]

已修改为平级，author 不管是 user 还是 bot，都视为 h2