Update .lintstagedrc configuration for new repo structure

In this commit we add a rule to the lint-staged configuration file
at the root of the repository to ensure that all the markdown files
contained in the docs/sources folder are formatted using our prettier
config.

Although not absolutely necessary, we enforce prettier to use the
markdown parser, and have it write the result directly to staged files.

.lintstagedrc

@@ -1,5 +1,9 @@
 {
+  "docs/sources/**/*.md": [
+    "prettier --parser markdown --write",
+    "eslint -c ./docs/sources/.eslintrc.js --fix"
+  ],
   "*.{js,jsx,css,scss}": "prettier --write",
   "*.{js,jsx}": "eslint --fix",
-  "*.{md,mdx}": "eslint -c ./src/data/markdown/.eslintrc.js --fix"
+  "src/data/markdown/**/*.{md,mdx}": "eslint -c ./src/data/markdown/.eslintrc.js --fix"
 }