Docs: netlify adapter site requirement (#2996)

* feat: human-readable error on bad site or base

* fix: human-readable error should have 1 config option

* docs: update README

* chore: changeset

* docs: mention localhost for testing via netlify CLI

.changeset/angry-suits-thank.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/netlify': patch
+---
+
+Add human-readable error when a site is not provided in your astro.config

packages/integrations/netlify/readme.md

@@ -2,14 +2,18 @@
 
 Deploy your server-side rendered (SSR) Astro app to [Netlify](https://www.netlify.com/).
 
-Use this adapter in your Astro configuration file:
+Use this adapter in your Astro configuration file, alongside a valid deployment URL:
 
 ```js
 import { defineConfig } from 'astro/config';
 import netlify from '@astrojs/netlify/functions';
 
 export default defineConfig({
-	adapter: netlify()
+	adapter: netlify(),
+  // Where your Netlify app will be deployed.
+  // Feel free to use a local URL (i.e. http://localhost:8080)
+  // to test local builds via the netlify CLI
+  site: 'https://my-production-url.netlify.app',
 });
 ```
 
@@ -23,7 +27,9 @@ netlify deploy
 
 ## Configuration
 
-The output folder is configuration with the `dist` property when creating the adapter.
+### dist
+
+We build to a `netlify` directory at the base of your project. To change this, use the `dist` option:
 
 ```js
 import { defineConfig } from 'astro/config';

packages/integrations/netlify/src/index.ts

@@ -28,7 +28,15 @@ function netlifyFunctions({ dist }: NetlifyFunctionsOptions = {}): AstroIntegrat
 				}
 			},
 			'astro:config:done': ({ config, setAdapter }) => {
-				setAdapter(getAdapter(new URL(config.base, config.site).toString()));
+				let site = null;
+				try {
+					site = new URL(config.base, config.site);
+				} catch {
+					throw new Error(
+						'The Netlify adapter requires a deployment URL. Ensure a "site" is specified in your astro.config. If you provided a "base" in your astro.config, ensure it is a valid path.'
+					);
+				}
+				setAdapter(getAdapter(site.toString()));
 				_config = config;
 			},
 			'astro:build:start': async ({ buildConfig }) => {