Clarify guidelines for selecting Chinese-related language codes

Chinese-related language codes look confusing because Chinese can be classified along multiple independent axes:

- By language / variety (e.g. Mandarin, Minnan, Yue, Wu, Hakka)
- By historical period (e.g. Classical Chinese vs. modern languages)
- By region or usage standard (e.g. Taiwan, Macao, Hong Kong, Mainland China, Singapore)
- By writing system (e.g. Traditional vs. Simplified characters, Latin-based romanization)

Wikidata preserves all of these distinctions, even when they overlap in everyday usage.
Therefore, we need to guide users toward consistent and meaningful choices within a flattened table structure.

To-do

- [x] Documentation / Guidelines
- [x] Add a “Language Code Guideline” section to WikiPage: Add A New Instrument Name
- [x] A language code in UMIL
- [x] Understanding the Chinese (Sinitic) language family in UMIL
- [x] Guideline to Selecting the Language Code for Sinitic Language Family
- [x] Update WikiPage: FAQ
- [x] Add: What is a language code in UMIL?
- [x] Add: Why are there multiple Chinese-related language codes?
- [x] Add: Why some language codes are not recommended
- [ ] UI: Add a link to the language code guideline in UMIL website
- [x] Location: “Add instrument name” button or help text nearby
- [x] Purpose: allow users to check guidance before selecting a code