Add sentence case linting rule

.remarkrc.mjs

@@ -1,4 +1,5 @@
 import remarkLintNoBlockedCharacters from './utils/plugins/remark/remark-lint-no-blocked-characters.mjs'
+import remarkLintSentenceCaseHeaders from './utils/plugins/remark/sentence-case-headers.mjs'
 
 export default {
   plugins: [
@@ -10,6 +11,7 @@ export default {
     "remark-lint-table-cell-padding",
     "remark-lint-table-pipe-alignment",
     "remark-lint-table-pipes",
+    remarkLintSentenceCaseHeaders,
     "@double-great/remark-lint-alt-text",
     [
       "remark-lint-heading-style",

utils/plugins/remark/sentence-case-headers.mjs

@@ -0,0 +1,47 @@
+import { visit } from 'unist-util-visit';
+import { lintRule } from 'unified-lint-rule';
+import { fileURLToPath } from 'url';
+import { dirname } from 'path';
+
+// Get the current directory
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+
+const remarkLintSentenceCaseHeaders = lintRule(
+  {
+    url: 'https://github.com/ethereum-optimism/docs', // Update with your actual URL
+    origin: 'remark-lint:sentence-case-headers'
+  },
+  (tree, file) => {
+    console.log("Linting headers...");
+
+    function sentenceCaseHeader(header) {
+      const text = header.children.map(node => node.value).join('').trim();
+      const words = text.split(' ');
+
+      // Capitalize the first word, make all others lowercase
+      const formatted = words.map((word, index) => {
+        if (index === 0) {
+          return word.charAt(0).toUpperCase() + word.slice(1).toLowerCase(); // First word capitalized
+        }
+        return word.toLowerCase(); // All other words lowercase
+      }).join(' ');
+
+      return formatted !== text ? formatted : null; // Return the formatted text if different
+    }
+
+    visit(tree, 'heading', (node) => {
+      const headerText = node.children.map(child => child.value).join('');
+      console.log(`Checking header: ${headerText}`);
+
+      const fixedText = sentenceCaseHeader(node);
+      if (fixedText) {
+        file.message('Header should be in sentence case', node);
+        node.children[0].value = fixedText; // Update the header text
+        file.message('Header text has been updated to sentence case', node);
+      }
+    });
+  }
+);
+
+export default remarkLintSentenceCaseHeaders;