docs: update readme to reflect current state of repo and update links

[jonathan-mayer]

Motivation

Finally switch to inviter.co from communityinviter since it will soon be deprecated.
And finally make people aware of the GoKubeDownscaler aswell as bringing the py-kube-downscaler into maintainance-only mode.

Changes

Tests done

TODO

before merging we should wait until:

• The GoKubeDownscalers Docusaurus page is done and actually deployed on main
• add docs for cosign verify

[github-actions]

File	Coverage
All files	97%	✅
test_autoscale_resource.py	99%	✅
test_cmd.py	100%	✅
test_events.py	100%	✅
test_grace_period.py	100%	✅
test_ignore_if_labels_dont_match.py	100%	✅
test_main.py	100%	✅
test_pods_force_uptime.py	100%	✅
test_resources.py	100%	✅
test_scaler.py	95%	✅
test_time.py	100%	✅

Minimum allowed coverage is 70%

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