Add quick-start-contribution-onboarding.rst and improve gitlab.rst

@kristis and I prepared documentation file which describes how to contribute to the Oniro Project

Closes #150 (closed)

Signed-off-by: Grzegorz Gwóźdź grzegorz.gwozdz@huawei.com

mentioned in issue #150 (closed)

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@ghassaneben the review is for you :)

@gwozdzcfs @kristis Ghassane has just arrived so he's the best person to review this doc :)

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• 470b23b9 - Add enumerated list in how-to-contribute.rst

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• 6f103fd3 - Add contributing/how-to-contribute.rst

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• 17b19a89 - 1 commit from branch eclipse/oniro-core:main
• e13d35b5 - Merge branch 'main' of gitlab.eclipse.org:eclipse/oniro-core/docs into ADD-HOW-TO-CONTRIBUTE

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@mrybczyn I've made same changes to the file