Cleanup docs on r1.13.0
Signed-off-by: Oleksii Kuchaiev <okuchaiev@nvidia.com>

# What does this PR do ?

Some fixes and minor improvements to README.rst and RTD docs,

# Changelog 

- [x] Moves Requirements and Installation into single source of truth (on RTD)
- [x] Cleans up README.rst to make it less verbose and removing redundant and obsolete links
  
**PR Type**:
- [ ] New Feature
- [ ] Bugfix
- [x] Documentation

