Reuse write_docs_to_subdir to generate to docs

Summary:
# This diff stack
Separate the rules at https://buck2.build/docs/prelude/globals/ into separate pages. Each page will have it's own page. With such change it will be more clean and it will be faster  to open the page.

# This diff

This diff reuse `write_docs_to_subdir` to generate to docs, with such change, we can generate seperate pages for each rule. (Will do this in later diff)

 This diff also handles the case where `name` is empty when generating documentation in `write_docs_to_subdir`. Previously, this would result in a meaningless empty `# ` line in the generated documentation.

Reviewed By: JakobDegen

Differential Revision: D73931901

fbshipit-source-id: 8601b922c5527a03a8725784881e359e9b5e1eb6

Author: Nero5023

starlark/src/docs/markdown.rs

@@ -180,7 +180,13 @@ pub(super) fn render_members<'a>(
     let member_details: Vec<_> = after_summary.into_iter().chain(member_details).collect();
     let members_details = member_details.join("\n\n---\n\n");
 
-    format!("# {name}{summary}\n\n{members_details}")
+    let header = if name == "" {
+        "".to_owned()
+    } else {
+        format!("# {name}")
+    };
+
+    format!("{header}{summary}\n\n{members_details}")
 }
 
 pub(super) fn render_doc_type(