Improve documentation

- added instruction about adding FVP executables into PATH environment
  variable
- removed leading space from markdown table which caused faulty view in
  Gitiles
- reworked description of run_platform.py script
- replaced SVG with PNG images to support rendering in Gitiles

Change-Id: I2f0b242138fff64b7ebc78f9ce6d76c3ef8a8e5b