Document usage of starlark DAP server

[lf-]

From #1032: it's currently not documented e.g. which protocol action needs to be set in the configuration to use nvim-dap or similar client with the starlark DAP. It does work though.

Here's a starting point for writing such docs: I mention a working config and usage procedure on https://github.com/mfussenegger/nvim-dap/issues/1534