docs: move markdown docs from .github/ to docs/

The GitHub guide on contributing file says: "Decide whether to store your
contributing guidelines in your repository's root, docs, or .github directory."

https://help.github.com/articles/setting-guidelines-for-repository-contributors/#adding-a-contributing-file

But there's really no advantage to keeping it in the hidden .github/, since
these are public and really belong together with the other documentation.

We can still keep the issue templates under .github/, since they are not really
documentation on their own.

Updated the links pointing to CONTRIBUTING.md to refer to the one in docs/.

.github/ISSUE_TEMPLATE/Bug_report.md

@@ -8,7 +8,7 @@ about: A report of an error in a recent systemd version
 > ...
 
 <!-- **NOTE:** Do not submit bug reports about anything but the two most recently released systemd versions upstream! -->
-<!-- For older version please use distribution trackers (see https://github.com/systemd/systemd/blob/master/.github/CONTRIBUTING.md#filing-issues). -->
+<!-- For older version please use distribution trackers (see https://github.com/systemd/systemd/blob/master/docs/CONTRIBUTING.md#filing-issues). -->
 
 **Used distribution**
  > …

docs/HACKING

@@ -9,7 +9,7 @@ Request (PR):
 Please make sure to follow our Coding Style when submitting patches. See
 docs/CODING_STYLE for details. Also have a look at our Contribution Guidelines:
 
-        https://github.com/systemd/systemd/blob/master/.github/CONTRIBUTING.md
+        https://github.com/systemd/systemd/blob/master/docs/CONTRIBUTING.md
 
 When adding new functionality, tests should be added. For shared functionality
 (in src/basic and src/shared) unit tests should be sufficient. The general