updating labels descriptions (#13557)

* updating labels descriptions

* delete milestones

* Update docs/CONTRIBUTING.adoc

Co-authored-by: Bastian Köcher <[email protected]>

* link to label docs

* Update docs/CONTRIBUTING.adoc

Co-authored-by: Oliver Tale-Yazdi <[email protected]>

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Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: parity-processbot <>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>