Nest config under mkdocs key.

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

plugins/techdocs-node/src/stages/generate/techdocs.test.ts

@@ -145,7 +145,7 @@ describe('readGeneratorConfig', () => {
           runIn: 'docker',
           dockerImage: 'my-org/techdocs',
           pullImage: false,
-          legacyCopyReadmeMdToIndexMd: true,
+          mkdocs: { legacyCopyReadmeMdToIndexMd: true },
         },
       },
     });

plugins/techdocs-node/src/stages/generate/techdocs.ts

@@ -222,7 +222,7 @@ export function readGeneratorConfig(
       'techdocs.generator.mkdocs.omitTechdocsCorePlugin',
     ),
     legacyCopyReadmeMdToIndexMd: config.getOptionalBoolean(
-      'techdocs.generator.legacyCopyReadmeMdToIndexMd',
+      'techdocs.generator.mkdocs.legacyCopyReadmeMdToIndexMd',
     ),
   };
 }