doc: introduce some minimalistic markdown without essential changes
The goal is to transform the standard documents
README, INSTALL, SUPPORT, CONTRIBUTING, ...
from a pure text format into markdown format, but in such a way
that the documentation remains nicely formatted an easy readable
when viewed with an normal text editor.
To achieve this goal, we use a special form of 'minimalistic' markdown
which interferes as little as possible with the reading flow.
* avoid [ATX headings][] and use [setext headings][] instead
(works for `<h1>` and `<h2>` headings only).
* avoid [inline links][] and use [reference links][] instead.
* avoid [fenced code blocks][], use [indented-code-blocks][] instead.
The transformation will take place in several steps. This commit
introduces mostly changes the formatting and does not chang the
content significantly.