Update the software documentation/manual for VTM/JVET
- change README.txt to markdown (displays in GitLab)
- update build instructions
- update LaTEX document template for JVET and include it
- general updates for the software manual (authors, purpose, etc.)
- update build instructions (aligned with README)
- use code highlighting (requires Pygments installation)
- minor formatting updates
- update copyright date in COPYING file
Edited by Karsten Suehring