Improve docs

[George-Miao]

I'm proposing a series of changes to improve our document quality. Here're things I'd like to do:

• Fix broken intra doc links (prohibit rustdoc::broken_intra_doc_links in future) (docs: fix intra doc links #554)
• Fix broken docs.rs builds (See broken builds) (docs: fix broken builds #562)
• Separate README.md for each subcrate
• Improve grammar and fluency in document

During the process, I think it will be a good chance to revisit some of the old code and make improvements to them as well (like #555). For anyone who's interested in involving, welcome to open a PR!