Add API links to the header bar

Summary:
Until we get new API infra for gh-pages, this will point to the C++ headers and Java
files, respectively for API information.

Test Plan:
Visual

https://www.facebook.com/pxlcld/pwzQ

Reviewers: lgalanis, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D63771

docs/_data/nav.yml

@@ -2,6 +2,14 @@
   href: /docs/
   category: docs
 
+- title: API (C++)
+  href: https://github.com/facebook/rocksdb/tree/master/include/rocksdb
+  category: api
+
+- title: API (Java)
+  href: https://github.com/facebook/rocksdb/tree/master/java/src/main/java/org/rocksdb
+  category: api
+
 - title: Support
   href: /support.html
   category: support