We now have comprehensive documentation available online on readthedocs.
Let's not duplicate docs in README - move all information into the
sphinx files.

While at it: make the remaining README use markdown.

Reviewed-by: Vincent Fazio <vfazio@xes-inc.com>
Link: https://lore.kernel.org/r/20250211-improve-docs-v4-17-dc56702c2ca8@linaro.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>