Fix link paths to work from root and /docs

[hondaya14]

Previously, "contribute to Kotlin/Native." (in contributing.md) link only worked under /docs. Now they work from the project root as well. No content changes.

As-Is

• NOT FOUND: kotlin/tree/master?tab=contributing-ov-file to kotlin/blob/kotlin-native/README.md (bad path)
• OK: kotlin/blob/master/docs/contributing.md to kotlin/blob/master/kotlin-native/README.md

To-Be

• OK: kotlin/tree/master?tab=contributing-ov-file to kotlin/blob/master/kotlin-native/README.md (ok path)
• OK: kotlin/blob/master/docs/contributing.md to kotlin/blob/master/kotlin-native/README.md

[hondaya14]

I’m sorry this PR isn’t associated with a YouTrack issue. If there’s one about updating documentation, please point me to it.