[lldb] Add a "design" section to the documentation.

Create a "Design" section for the LLDB documentation. The goal is to
have design documents that describe how the LLDB internals work.

Currently similar pages  are mixed together under the "Development". The
existing pages describing the architecture, the reproducers, the
structured data plugins, and the SB API could be housed here. I hope
we'd see more pages being added here in the future.

Differential revision: https://reviews.llvm.org/D88516