Change markdown images to html images

[TheDauntless]

This PR changes the following:

• Modify markdown images to HTML images as per Remove unnecessary backslash #1979
• Slightly modify the build process so that it works on files in the build folder rather than the original files
• Make the sed replace pattern for images tolerant for whitespace

[cpholguera]

Thanks a lot @TheDauntless. Please take a look at the comment. I'm not sure if the commented changes were really intended.

[cpholguera]

this is what I could still catch :D

[cpholguera]

Awesome, thanks a lot @TheDauntless!