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

[David Blaikie via llvm-commits]

dwblaikie wrote:

> About the words "DWARF", "CodeView", and "PDB", I wasn't sure whether to treat them as "code" (and put in monospace) or just leave them alone without styling... Anyway, probably best to choose one approach for such words and use it consistently throughout.

In general I'd say all three appear often enough and refer to a concept, that I wouldn't treat them as code identifiers - keep them as plain english language identifiers. (eg: I think it'd be weird if the DWARF spec used monospaced for every appearance of the word 'DWARF' in the spec)

https://github.com/llvm/llvm-project/pull/86394