docs: add v20.0.0 release notes, archive v19.3.0 and drop partial ones (dashpay#5674)

## Issue being fixed or feature implemented
NOTE: this PR is for v20.x branch, to be merged as the last one before
v20 release.

## What was done?


## How Has This Been Tested?


## Breaking Changes

## Checklist:
- [ ] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

Author: UdjinM6