fix internal links for remote substream docs (#371)

fix remote links

Author: Dimitri POSTOLOV

website/pages/en/substreams/[[...slug]].mdx

@@ -36,6 +36,18 @@ export async function getStaticProps({ params: { slug = ['README'] } }) {
               }
             }
           })
+          visit(tree, 'link', (node) => {
+            if (node.url.startsWith('./')) {
+              node.url = node.url.slice(2)
+            }
+            if (node.url.startsWith('/')) {
+              // (foo)[/foo/bar]
+              node.url = node.url.replace('/', '/substreams/')
+            } else if (!node.url.includes('/') && node.url.endsWith('.md')) {
+              // (foo)[foo.md]
+              node.url = [...slug.slice(0, -1), node.url].join('/')
+            }
+          })
           done()
         },
       ],