docs(concepts): clarify zero-config entry default and error message

adds a tip to concepts/index.mdx and entry-points.mdx explaining that the
zero-config default entry is './src/index.js', and why the error message
references './src' rather than './src/index.js'.

closes #2035

Author: saishankar404

src/content/concepts/entry-points.mdx

@@ -12,6 +12,7 @@ contributors:
   - adyjs
   - anshumanv
   - ritikbanger
+  - saishankar404
 ---
 
 As mentioned in [Getting Started](/guides/getting-started/#using-a-configuration), there are multiple ways to define the `entry` property in your webpack configuration. We will show you the ways you **can** configure the `entry` property, in addition to explaining why it may be useful to you.
@@ -25,6 +26,8 @@ The result of this process is later written to disk according to the
 `output` configuration, which controls where and how the bundled
 files are emitted.
 
+T> When running webpack without a configuration file, the entry defaults to `'./src/index.js'`. If that file is missing — even if your `src/` directory exists — webpack will throw: <br /> **`ERROR in Entry module not found: Error: Can't resolve './src'`** <br /><br /> The error says `'./src'` rather than `'./src/index.js'` because webpack resolves the directory first, then fails to find `index.js` inside it. To use a different entry filename, configure the `entry` option as shown in the sections below.
+
 ## Single Entry (Shorthand) Syntax
 
 Usage: `entry: string | [string]`

src/content/concepts/index.mdx

@@ -20,6 +20,7 @@ contributors:
   - muhmushtaha
   - chenxsan
   - RyanGreyling2
+  - saishankar404
 ---
 
 At its core, **webpack** is a _static module bundler_ for modern JavaScript applications. When webpack processes your application, it internally builds a [dependency graph](/concepts/dependency-graph/) from one or more _entry points_ and then combines every module your project needs into one or more _bundles_, which are static assets to serve your content from.
@@ -59,6 +60,8 @@ export default {
 };
 ```
 
+T> When running webpack without a configuration file, the entry defaults to `'./src/index.js'`. If that file does not exist — even if your `src/` directory does — webpack will throw: <br /> **`ERROR in Entry module not found: Error: Can't resolve './src'`** <br /><br /> The error says `'./src'` rather than `'./src/index.js'` because webpack resolves the directory first and then fails to find the default `index.js` inside it. To use a different entry filename, add a `webpack.config.js` as shown in the example above.
+
 T> Learn more in the [entry points](/concepts/entry-points) section.
 
 ## Output