Update contribution guide

[gztensor]

Description

Add PR cycle diagram and requirement to base changes on devnet-ready.

Related Issue(s)

n/a

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation update
• [ ] Other (please describe):