Make it easier to see warnings and errors

**Operating System**: Windows

**DocFX Version Used**: 2.56.6.0

**Template used**: `default`

**Steps to Reproduce**:

1. Write some invalid markdown that will generate a warning or an error.
2. Run docfx.exe to generate documentation.
3. View the log file.

**Actual Behavior**:
First, depending on the amount of documentation being generated and the verbosity of the log, it is difficult to find warnings and errors without using a text editor with a "find all in file" feature. My suggestion would be to allow options for outputting all warnings and errors to separate log files (while keeping the full log file).

Second, it would be very helpful to have the line number included in the warning or error message in the log file. Currently the warning/error message only reports the file. This makes it more difficult to find where the actual warning/error is, especially if it is a rather large file.