new Contributing to the Documentation section

Summary:
This is a new section under 'Contributing to the Documentation', in the 'Under the Hood' menu.

It includes two new pages:

* Documentation Formatting Tips (created here.  For content, see D37006489)
* Writing Guide (created here.  For content, see D37006980)

Setup in sidebar.js.

Reviewed By: lblasa

Differential Revision: D37004092

fbshipit-source-id: 5fa0a38632f7fa9d9178b69a89d9b21f5bda39ab

docs/internals/documentation-formatting.mdx

@@ -0,0 +1,5 @@
+---
+id: documentation-formatting
+title: Formatting Tips
+---
+import useBaseUrl from '@docusaurus/useBaseUrl';

docs/internals/documentation-writing-guide.mdx

@@ -0,0 +1,5 @@
+---
+id: documentation-writing-guide
+title: Writing Guide
+---
+import useBaseUrl from '@docusaurus/useBaseUrl';