Fix: Improve TS Warning For http_api_reference.md (#8172)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/8157
The current master code should work fine, but hI ave some warnings, so I
added a declare to improve the warning

### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/custom.d.ts

@@ -0,0 +1,4 @@
+declare module '*.md' {
+  const content: string;
+  export default content;
+}