[Docs] Generate the LLDB man page with Sphinx

This patch replaces the existing out-of-date man page for lldb and
replaces it with an RST file from which sphinx generates the actual
troff file. This is similar to how man pages are generated for the rest
of the LLVM utilities.

The man page is generated by building the `docs-lldb-man` target.

Differential revision: https://reviews.llvm.org/D70514