Documentation Request: Active Debugger instruction when debugging Mill codebase

[Friendseeker]

Mill's official documentation says:

mill/readme.adoc

Lines 91 to 97 in d219462

	In general, `println` or `pprint.log` should work in most places and be sufficient for
	instrumenting and debugging the Mill codebase. In the occasional spot where `println`
	doesn't work you can use `mill.main.client.DebugLog.println` which writes to a file
	`~/mill-debug-log.txt` in your home folder. `DebugLog` is useful for scenarios like
	debugging Mill's terminal UI (where `println` would mess things up) or subprocesses
	(where stdout/stderr may get captured or used and cannot be used to display your own
	debug statements).

While println debugging works for many people, it would be ideal to also provide instruction for active debugger usage, for people that prefers the latter.

For extra context, instructions online to use active debugger for a Mill project (e.g. on Stackoverflow) are incomplete (or maybe outdated). I tried to follow the instruction on aforementioned Stackoverflow post when running mill.main.maven.BuildGenTests, the debug port is never opened.

One may still use BSP for active debugging, by running the target directly via IDE. However BSP debugging support seems to be incomplete. For instance, it does not set MILL_TEST_RESOURCE_DIR. We may want to consider documenting BSP as an option & mentioning its caveats.

java.util.NoSuchElementException: MILL_TEST_RESOURCE_DIR
    at scala.sys.package$.$anonfun$env$1(package.scala:69)
    at scala.collection.immutable.Map$WithDefault.default(Map.scala:181)