docs: add tsconfig.json support details to TypeScript page (#3026)

Author: bartlomieju

runtime/fundamentals/typescript.md

@@ -264,6 +264,46 @@ To specify the library files to use in a TypeScript file, you can use
 /// <reference lib="dom" />
 ```
 
+## Configuring TypeScript with `tsconfig.json` {#tsconfig}
+
+While Deno uses `deno.json` for TypeScript configuration by default, it also
+supports `tsconfig.json` for compatibility with existing Node.js and TypeScript
+projects, making it easier to adopt Deno incrementally. Each workspace directory
+containing a `deno.json` or `package.json` is probed for a `tsconfig.json` — if
+one exists, Deno will automatically use it for type checking and the language
+server, no extra flags needed. Since Deno 2.1, `jsconfig.json` is also
+auto-detected when a `package.json` is present, which is useful for
+JavaScript-only projects.
+
+For example, an existing Node.js project with this `tsconfig.json`:
+
+```json title="tsconfig.json"
+{
+  "compilerOptions": {
+    "strict": true,
+    "jsx": "react-jsx",
+    "lib": ["dom", "esnext"],
+    "paths": {
+      "@/*": ["./src/*"]
+    }
+  },
+  "include": ["src/**/*"]
+}
+```
+
+will be picked up automatically when running `deno check` or using the Deno
+language server. If a `deno.json` with its own `compilerOptions` is added later,
+those take precedence.
+
+:::tip
+
+For Deno-first projects, prefer `compilerOptions` in `deno.json` over a separate
+`tsconfig.json`. See
+[Configuring TypeScript](/runtime/reference/ts_config_migration/) for the full
+list of supported fields, precedence rules, and compiler option defaults.
+
+:::
+
 ## Augmenting global types
 
 Deno supports ambient or global types in TypeScript. This is useful when