Fix: Move GitHub-specific documents into .github

[localheinz]

Description

As lined out at https://github.com/blog/2111-issue-and-pull-request-templates:

If you're worried about the added clutter in the root directory of your project, we also added support for a .github/ folder. You can put CONTRIBUTING.md, ISSUE_TEMPLATE.md, and PULL_REQUEST_TEMPLATE.md files in .github/ and everything will work as expected.

this PR moves files specific to GitHub into .github/.

💁‍♂️ I am unaware whether these changes affect any external pages linking to these documents, but these changes will definitely work fine here on GitHub.

Fixed Issues (if relevant)

n/a

Manual testing scenarios

n/a

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[orlangur]

Great! This item was in my TODO list since I saw the corresponding GitHub blog post (or maybe it was some angular repo?).

But there is small thing to do yet. Please grep all over the code or these files occurrences and change them accordingly (at least there were some in root .htaccess) and make this new folder inaccessible like https://github.com/magento/magento2/blob/develop/vendor/.htaccess (or maybe dot folders are restricted by default? don't have Apache in my hand to check...)

[localheinz]

Thank you, @magento-team and @orlangur!