[lldb-dap] Separate user and developer documentation (#92428)

The README.md is what users see when they look for the extension in the
Marketplace [1]. Right now, it's a mix of developer documentation (for
us) and user documentation. This commit moves the developer docs into
`docs` and the lldb website and refocuses the README on using the
extension.

[1] https://marketplace.visualstudio.com/items?itemName=llvm-vs-code-extensions.lldb-dap