Refa: refactor prompts into markdown-style structure using Jinja2 (#8667)

### What problem does this PR solve?

Refactor prompts into markdown-style structure using Jinja2.

### Type of change

- [x] Refactoring

rag/prompts/keyword_prompt.md

@@ -0,0 +1,16 @@
+## Role
+You are a text analyzer.
+
+## Task
+Extract the most important keywords/phrases of a given piece of text content.
+
+## Requirements
+- Summarize the text content, and give the top {{ topn }} important keywords/phrases.
+- The keywords MUST be in the same language as the given piece of text content.
+- The keywords are delimited by ENGLISH COMMA.
+- Output keywords ONLY.
+
+---
+
+## Text Content
+{{ content }}