Update README.md instructions for running doxygen

[jafingerhut]

No description provided.

[jafingerhut]

@AdarshRawat1 These changes make sense to me regarding my confusions about running doxygen. Let me know if you see anything in these changes that is different than what you would recommend.

In particular, if we want to recommend using cmake with -DENABLE_DOCS=ON, it would be good to make those instructions work. Right now all of my attempts to use those steps have resulted in an error message, as mentioned in this issue: #4805

[fruffy]

Conditional approval pending @AdarshRawat1 's review.

[jafingerhut]

@fruffy I am guessing the one failing test has nothing to do with changes to the README.md. I am leaving it there for now, in case you want to examine it and see if its root cause looks familiar to you. I do not know why it occurred.

[fruffy]

Looks like a racecondition to me. Not quite clear why the Python lex/yacc code is sharing state here.

[jafingerhut]

@AdarshRawat1 Could you please take a look at these changes and give any comments, or suggestions for further changes, that you think of?

[AdarshRawat1]

@AdarshRawat1 Could you please take a look at these changes and give any comments, or suggestions for further changes, that you think of?

Since we only have a stable configuration for HTML output, these changes make total sense.