[clang][docs][dataflow] Added an introduction to dataflow analysis

This documentation supports the dataflow analysis framework (see "[RFC]
A dataflow analysis framework for Clang AST" on cfe-dev).

Since the implementation of the framework has not been committed yet,
right now the doc describes dataflow analysis in general.

Since this is the first markdown document in clang/docs, I added support
for Markdown to clang/docs/conf.py in the same way as it is done in
llvm/docs.

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D114231