Merge "docs(auth): add more information about CoTs" into integration

docs(auth): add more information about CoTs

Explain that platforms are free to define their own Chain of Trust (CoT)
based on their needs but default ones are provided in TF-A source code:
TBBR, dua

docs(auth): add more information about CoTs

Explain that platforms are free to define their own Chain of Trust (CoT)
based on their needs but default ones are provided in TF-A source code:
TBBR, dualroot and CCA.

Give a brief overview of the use case for each of these CoTs.

Simplified diagrams are also provided for the TBBR and dualroot CoTs -
CCA CoT is missing such a diagram right now, it should be provided as a
future improvement.

Also do some cosmetic changes along the way.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: I7c4014d4d12d852b0ae5632ba9c71a9ad266080a

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