perf(v2): improve dev build time by not overwriting file if possible (#2089)

* perf(v2): improve sequential build time by not overwriting file if possible

* minor improvement

* docs

packages/docusaurus-utils/src/index.ts

@@ -27,7 +27,19 @@ export async function generate(
     return;
   }
 
-  const lastHash = fileHash.get(filepath);
+  let lastHash = fileHash.get(filepath);
+
+  // If file already exist but its not in runtime cache hash yet,
+  // we try to calculate the content hash and then compare
+  // This is to avoid unnecessary overwrite and we can reuse old file
+  if (!lastHash && fs.existsSync(filepath)) {
+    const lastContent = await fs.readFile(filepath, 'utf8');
+    lastHash = createHash('md5')
+      .update(lastContent)
+      .digest('hex');
+    fileHash.set(filepath, lastHash);
+  }
+
   const currentHash = createHash('md5')
     .update(content)
     .digest('hex');

website/docs/markdown-features.mdx

@@ -183,6 +183,7 @@ For example, if you are in `doc2.md` and you want to reference `doc1.md` and `fo
 
 ```md
 I am referencing a [document](doc1.md). Reference to another [document in a folder](folder/doc3.md)
+[Relative document](../doc2.md) referencing works as well.
 ```
 
 One benefit of this approach is that the links to external files will still work if you are viewing the file on GitHub.