spdx: Improve content of FAQ and Limitations sections

* Decouple parsing from generation
* Improve name of internal variables

- Sort import alphabetically
- Move script documentation at the script/module level
  and improve help display

This commit updates the "SPDX_generate_output.py" script to accept
response files as input source file specifications. The previous
implementation had a limitation of a maximum command length of 8191
characters on a Windows system (as documented in [1]), which led
to generation failures in some modules. By using response files for
specifying input files, this commit fixes the issue.

In particular, certain modules such as the one listed below
needed to accept input files beyond the 8191 limit:
- vtkCommonImplicitArrays
- vtkCommonDataModel
- vtkCommonCore
- vtkRenderingOpenGL2
- vtkInteractionWidgets

To demonstrate this, we list these modules and the length of their
"-spdx.Release.args" files using the commands shown below:

```
$ find . -type f -name "*-spdx.Release.args" | xargs wc -m | sort -n -r | tail +2 | head -n6
 52896 ./vtkCommonImplicitArrays-spdx.Release.args
 13542 ./vtkCommonDataModel-spdx.Release.args
 10576 ./vtkCommonCore-spdx.Release.args
  9783 ./vtkRenderingOpenGL2-spdx.Release.args
  9302 ./vtkInteractionWidgets-spdx.Release.args
  6592 ./vtkFiltersGeneral-spdx.Release.args
```

This listed was generated assuming the VTK source and build directories
are respectively located at "C:/VTK" and "C:/VTK-build".

This approach is similar to the one used for passing input files
to the Python wrapping tool (see `CMake/vtkModuleWrapPython.cmake`
that was originally implemented in 328d974b "ENH: Allow wrapping
tools to accept argument files.")

[1] https://learn.microsoft.com/en-us/troubleshoot/windows-client/shell-experience/command-line-string-limitation

To avoid the apostrophe (or single quote) from confusing the execution
of the command associated the build-system specific code resulting from
the use of "add_custom_command()", this commit use the recommended VERBATIM
option and ensure the argument is space separated.

This allows to specify name like "John O'Keefe" or "Bob O'Bara"

It turns out that the introduction of single-quote for handling
argument passed to "SPDX_generate_output.py" allows to support the
of parentheses.

Experiment done using the "Unix Makefiles" generator also confirmed
that the single escaping had no effect.