chore: fix path and samples for lexical-mdx tests (#9433)

Later we could add better examples that test complex use cases, but at
least this allows the code to compile.

test/lexical-mdx/collections/Posts/shared.ts

@@ -1,5 +1,10 @@
-export const docsBasePath = '/Users/alessio/Documents/payloadcms-mdx-mock/docs'
+import path from 'path'
+import { fileURLToPath } from 'url'
 
+export const docsBasePath =
+  typeof window === 'undefined'
+    ? path.resolve(path.dirname(fileURLToPath(import.meta.url)), '../../sampleDocs')
+    : null
 export const languages = {
   ts: 'TypeScript',
   plaintext: 'Plain Text',

test/lexical-mdx/sampleDocs/test.mdx

@@ -0,0 +1,3 @@
+# You can write MDX here
+
+Write whatever content you want for testing purposes