feat: add "Help" to sidebar

[taoky]

Related: #104, #90 (not the same as it)

Deps:

• https://github.com/mirrorz-org/mirrorz-i18n/pull/2
• https://github.com/mirrorz-org/mirrorz-config/pull/8

Bugs:

• [ ] Does not work in legacy pages legacy 不处理，因为 help 也没有做 legacy 有关支持

Screenshots (with open-in-new symbol):