Added Contribution Guide

Added Contribution Guide

Exhaustive Contribution Guide. Removed Trailing Whitespaces.
Updated index.md. Abandoned accidental commits 25474,
25474 and uploading a new patchset. Corrected code renderings.

v2 (jsmoeller): Update contribution subchapters

Bug-AGL: SPEC-3633

Signed-off-by: Shankho Boron Ghosh <shankhoghosh123@gmail.com>
Change-Id: Ia808adc0208052f0591396860ac776e34070d279
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25473
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
(cherry picked from commit de81a8b07f8139fa0f65dddffc839b4a5e83dad7)
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25393