Add a web page for meetup

Summary:
By following
https://www.facebook.com/groups/629866953728590/permalink/629872443728041/,
add a web page for meetup information at rocksdb.or/meetup

Test Plan:
Locally loaded: https://www.facebook.com/pxlcld/l15s

The header is a bit weird. That's my browser's issue. I use the same
header as existing pages.

Reviewers: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16329