doc: rewrite and restructure doc/P4sim/README.md (#722)

* doc: update P4sim README with cleaner structure and content

Signed-off-by: Mingyu Ma <mingyu.ma@tu-dresden.de>

* doc: update P4sim README with installation guide

Signed-off-by: mingyuma <mingyu.ma@tu-dresden.de>

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Signed-off-by: Mingyu Ma <mingyu.ma@tu-dresden.de>
Signed-off-by: mingyuma <mingyu.ma@tu-dresden.de>

Author: Mingyumaz