docs: document recompiling V itself with debug info, debugging tests, and a tip for vscode (#10358)

doc/docs.md

@@ -3860,6 +3860,11 @@ This will make V produce the `file.c` then stop.
 If you want to see the generated C source code for *just* a single C function,
 for example `main`, you can use: `-printfn main -o file.c`.
 
+To debug the V executable itself you need to compile from src with `./v -g -o v cmd/v`.
+
+You can debug tests with for example `v -g -keepc prog_test.v`. The `-keepc` flag is needed, 
+so that the executable is not deleted, after it was created and ran.
+
 To see a detailed list of all flags that V supports,
 use `v help`, `v help build` and `v help build-c`.
 

doc/vscode.md

@@ -81,6 +81,9 @@ or create the file `.vscode/launch.json`:
 }
 ```
 
+**Optional:** use `"program": "${fileDirname}/${fileBasenameNoExtension}"` to debug 
+any current open source file with an existing binary with the same name but without any extention.
+
 ### Usage
 
 To allow your compiled application to be debugged.