Remove .html on all docs urls

Now that Flarum docs have been moved to docusaurus, URLs no longer end with `.html`.

Closes https://github.com/flarum/core/issues/3092

src/Extend/ServiceProvider.php

@@ -21,7 +21,7 @@ class ServiceProvider implements ExtenderInterface
      *
      * Service providers are an advanced feature and might give access to Flarum internals that do not come with backward compatibility.
      * Please read our documentation about service providers for recommendations.
-     * @see https://docs.flarum.org/extend/service-provider.html
+     * @see https://docs.flarum.org/extend/service-provider/
      *
      * @param string $serviceProviderClass The ::class attribute of the service provider class.
      * @return self

src/Install/Prerequisite/WritablePaths.php

@@ -53,7 +53,7 @@ class WritablePaths implements PrerequisiteInterface
             })->map(function ($path, $index) {
                 return [
                     'message' => 'The '.$this->getAbsolutePath($path).' directory is not writable.',
-                    'detail' => 'Please make sure your web server/PHP user has write access to this directory'.(in_array($index, $this->wildcards) ? ' and its contents' : '').'. Read the <a href="https://docs.flarum.org/install.html#folder-ownership">installation documentation</a> for a detailed explanation and steps to resolve this error.'
+                    'detail' => 'Please make sure your web server/PHP user has write access to this directory'.(in_array($index, $this->wildcards) ? ' and its contents' : '').'. Read the <a href="https://docs.flarum.org/install/#folder-ownership">installation documentation</a> for a detailed explanation and steps to resolve this error.'
                 ];
             });
     }