docs: update `CONTRIBUTING.md`

Created by: valerybugakov

Context

To unblock the community team Hacktoberfest announcement, this PR adds missing pieces of the documentation required for external contributors. It's the second iteration on the CONTRIBUTING.md related to this effort.

Changes

• Left only general guidelines about project setup in the CONTRIBUTING.md.
• Extracted frontend related instructions into a separate document in our developer documentation.
• Added a section with recommended CI checks to run locally for frontend issues.
• Linked to the general "Getting started" guide in case of the backend issue. We can improve this aspect later.