docs: fixed broken DEVELOPER.md links

[thomaspink]

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Other... Please describe: Documentation

What is the current behavior? (You can also link to an open issue here)
DEVELOPER.md links in CONTRIBUTING.md are broken as the docs/ folder is missing.
This problem may have happend because CONTRIBUTING.md was moved back to the root:
docs: revert the move of CONTRIBUTING.md to docs/CONTRIBUTING.md

What is the new behavior?
Fixed DEVELOPER.md links in CONTRIBUTING.md

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

[IgorMinar]

thanks

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