build(docs): fix `pnpm run docs:{build,verify}`

Our docs script was searching for docs in the src/docs folder,
which no longer exists.

Instead, it should be looking at the packages/mermaid/src/docs folder.

packages/mermaid/src/docs.mts

@@ -41,7 +41,9 @@ import { remark } from 'remark';
 // @ts-ignore No typescript declaration file
 import flatmap from 'unist-util-flatmap';
 
-const SOURCE_DOCS_DIR = 'src/docs';
+// These paths are from the root of the mono-repo, not from the
+// mermaid sub-directory
+const SOURCE_DOCS_DIR = 'packages/mermaid/src/docs';
 const FINAL_DOCS_DIR = 'docs';
 
 const AUTOGENERATED_TEXT = `# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT. Please edit the corresponding file in ${SOURCE_DOCS_DIR}.`;