Introduce a new HACKING.md document to replace the previous HACKING

Summary:
This document describes my own practices on how I develop and work on
KWin. Since we have nested KWin/Wayland my ways of developing completely
changed. Especially since it's possible to run without installing. Due
to that I thought it's a good idea to document how one can easily build
and run KWin.

Why a new document in the source tree instead of the wiki?

The main reason for using a wiki was that it allows to format, to link,
etc. This was prior to markdown and prior to code hosting places like
github or phabricator rendering the markdown pages. Given that I think
the information belongs in the repository and not in an external wiki.

Why not fix the existing page?

Our documentation on the wiki is quite outdated:

• Qt 4 based
• only X11
• reviewboard
• information about file structure which does not match reality

Especially the last point is where I think we need to do a clean cut and
just drop such information. Documentation about code belongs in the code
and not detached in wiki. It will outdate.

How to prevent this document will not become outdated?

Of course difficult. But I tried to not describe anything which might
change, such as file layout, etc.

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17600