doc: introduce some minimalistic markdown without essential changes

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.

[ATX headings]:         https://github.github.com/gfm/#atx-headings
[setext headings]:      https://github.github.com/gfm/#setext-headings
[inline links]:         https://github.github.com/gfm/#inline-link
[reference links]:      https://github.github.com/gfm/#reference-link
[fenced code blocks]:   https://github.github.com/gfm/#fenced-code-blocks
[indented code blocks]: https://github.github.com/gfm/#indented-code-blocks

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10545)