feat: add new api "notebook"

[InertGas01]

> curl -X GET http://127.0.0.163:8000/notebook/User_Guide
Shows the content of note "User_Guide".
"/<Title>/ Show the content of the note with title <Title>."

> curl -X POST http://127.0.0.163:8000/notebook/new_note_title/Content
If note "new_note_title" doesn't exist, create it with content "Content".
Otherwise, change the content of note "new_note_title" from "content" to "Content".

> curl -X POST http://127.0.0.163:8000/notebook/new_note_title/append/can't
> curl -X POST http://127.0.0.163:8000/notebook/new_note_title/append/contain
> curl -X POST http://127.0.0.163:8000/notebook/new_note_title/append/whitespaces.
Append " can't", " contain", " whitespaces." respectively to the content of note "new_note_title".

> curl -X GET http://127.0.0.163:8000/notebook/new_note_title
Shows the content of note "new_note_title".
"Content can't contain whitespaces."

[andy89923]

Rebase branch and remove this line.

[andy89923]

Rebase branch and remove this line.