docs: restore stable heading anchors and external links

- Restore the original `## Environment Variables`, `## PHP config`, and
  `## Hardening Images` headings. Those were already specific in
  context, and renaming them broke inbound anchor links from README.md
  and from migrate.md, performance.md.
- Update the worker.md cross-link to symfony.md to point at the new
  `## Symfony Worker Mode with FrankenPHP` anchor.
- Restore the original developer.chrome.com link for the 103 Early
  Hints reference. The MDN URL swap was an unrequested side-edit.

Author: dunglas

docs/config.md

@@ -290,7 +290,7 @@ But when the fix depends on a third party you don't control,
 }
 ```
 
-## FrankenPHP Environment Variables
+## Environment Variables
 
 The following environment variables can be used to inject Caddy directives in the `Caddyfile` without modifying it:
 
@@ -303,7 +303,7 @@ As for FPM and CLI SAPIs, environment variables are exposed by default in the `$
 
 The `S` value of [the `variables_order` PHP directive](https://www.php.net/manual/en/ini.core.php#ini.variables-order) is always equivalent to `ES` regardless of the placement of `E` elsewhere in this directive.
 
-## PHP Configuration in FrankenPHP
+## PHP config
 
 To load [additional PHP configuration files](https://www.php.net/manual/en/configuration.file.php#configuration.file.scan),
 the `PHP_INI_SCAN_DIR` environment variable can be used.

docs/docker.md

@@ -210,7 +210,7 @@ The Docker images are built:
 - when a new release is tagged
 - daily at 4 am UTC, if new versions of the official PHP images are available
 
-## Hardening FrankenPHP Docker Images
+## Hardening Images
 
 To further reduce the attack surface and size of your FrankenPHP Docker images, it's also possible to build them on top of a
 [Google distroless](https://github.com/GoogleContainerTools/distroless) or

docs/early-hints.md

@@ -5,7 +5,7 @@ description: FrankenPHP natively supports the HTTP 103 Early Hints status code,
 
 # Early Hints
 
-FrankenPHP natively supports the [103 Early Hints status code](https://developer.mozilla.org/docs/Web/HTTP/Reference/Status/103).
+FrankenPHP natively supports the [103 Early Hints status code](https://developer.chrome.com/blog/early-hints/).
 Using Early Hints can improve the load time of your web pages by 30%.
 
 ```php

docs/worker.md

@@ -44,7 +44,7 @@ This feature is often used in combination with [hot reloading](hot-reload.md).
 
 ## Worker Mode for Symfony
 
-See [the FrankenPHP Symfony worker mode documentation](symfony.md#worker-mode).
+See [the FrankenPHP Symfony worker mode documentation](symfony.md#symfony-worker-mode-with-frankenphp).
 
 ## Worker Mode for Laravel Octane