Documenting emulator debugging process with GDB (#1415)

* Added emulator debugging with GDB documentation

This documents the process of debugging RISC-V programs against the GNU debugger with OpenOCD.
https://github.com/freechipsproject/rocket-chip/issues/1339

* Review 1 by Megan.

review 1

* The executable arg is a dummy arg when using RBB emulator

* testing programs on emulator + reducing size ideas

+ Pointed out that {more size --> more execution time} and a solution.
+ Separated simple programs execution and testing (step 2) of program debugging (step 3): To illustrate the fact that custom programs can be run on the emulator too not only the benchmarks.

* typo minor change

* Modifications

Added an example command of VCD output + log file generation
Rearranged VCD size idea into the part where I talk about executing/testing
Cleaned redundancies