[lldb][docs] Update landing page for monorepo

Summary: Following up from D65330, here's an update for the landing page.

Reviewers: jryans, clayborg, amccarth, labath

Reviewed By: jryans, amccarth, labath

Subscribers: arphaman, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65437

llvm-svn: 367539