Fix poor doxygen indentation choices.

[bangerth]

From the comment in the program:

        # Then print the line. doxygen has the annoying habit of eating
        # the maximal number of spaces at the front of each code block,
        # leading to visually wrong indentation if one, for example, has
        # ```
        #   if (cond)
        #   {
        # ```
        # in one block, and then
        # ```
        #     some_function();
        # ```
        # in the next block -- the call to some_function() is not shown any
        # further to the right than the if(cond) before. Work around this by
        # prefixing all code lines with a non-printing Unicode space 0x00A0
        # that doxygen interprets as the first non-space character for
        # determining indentation, but that does not actually print as anything
        # other than a space (and that compilers appear to successfully ignore
        # when one copy-pastes code snippets from the generated doxygen pages
        # into an editor).

Visually, this looks as follows:

Note the poor indentation of the second and third code blocks. This patch fixes this.

[bangerth]

With the patch:

[tamiko]

@bangerth This was annoying me for a long time, thanks for the fix!

[bangerth]

Different kinds of whitespace for the win! Which of course brings up good memories of this paper: https://www.stroustrup.com/whitespace98.pdf