Move DCO into docs section

The DCO currently resides in an own directory creating a pointless
additional click/tab in order to reach end read it. It also just
clutteres the directory structure of the project.

Therefore this patch provides moves the DCO into an own legal section in
the docs directory, which is hopefully a more reasonable place.

This section can also be extended in future in order to host other legal
documents as well.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

CONTRIBUTING.md

@@ -23,7 +23,7 @@ We use the Developer Certificate of Origin (DCO) as a additional safeguard
 for the CodiMD project. This is a well established and widely used
 mechanism to assure contributors have confirmed their right to license
 their contribution under the project's license.
-Please read [contribute/developer-certificate-of-origin][dcofile].
+Please read [docs/legal/developer-certificate-of-origin.txt][dcofile].
 If you can certify it, then just add a line to every git commit message:
 
 ````