[llvm-debuginfo-analyzer][DOC] Convert 'README.txt' to markdown. (#86394)

As part of the WebAssembly support work
https://github.com/llvm/llvm-project/pull/85566

The README.txt is a bit odd since it only lists issues and problems
without talking about what works. It’s also hard to read on the GitHub
web view.

- Convert to Markdown and linking to the command docs
https://llvm.org/docs/CommandGuide/llvm-debuginfo-analyzer
- Rename some left 'elf reader' to 'DWARF reader'.