docs: spec updates for arbitrary blob signing (#811)

CLI Spec updated for Arbitrary blob signing. Proposal
https://hackmd.io/ewbJr2ZnT4a8U1ObDVXcSw?view#CLI-Spec and
https://hackmd.io/@-KPyDkW6QfGA-pldFa13pA/ByuHffALa

Signing Scheme and trust policy updates :
notaryproject/specifications#283

Signed-off-by: rgnote <[email protected]>

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Signed-off-by: rgnote <[email protected]>
Signed-off-by: Patrick Zheng <[email protected]>
Signed-off-by: Feynman Zhou <[email protected]>
Signed-off-by: dependabot[bot] <[email protected]>
Signed-off-by: Cameron Rozean <[email protected]>
Signed-off-by: Yi Zha <[email protected]>
Signed-off-by: Rakesh Gariganti <[email protected]>
Signed-off-by: Junjie Gao <[email protected]>
Signed-off-by: Toddy Mladenov <[email protected]>
Co-authored-by: Patrick Zheng <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Feynman Zhou <[email protected]>
Co-authored-by: Cameron Rozean <[email protected]>
Co-authored-by: Yi Zha <[email protected]>
Co-authored-by: Shiwei Zhang <[email protected]>
Co-authored-by: Milind Gokarn <[email protected]>
Co-authored-by: Junjie Gao <[email protected]>
Co-authored-by: Toddy Mladenov <[email protected]>
Co-authored-by: Pritesh Bandi <[email protected]>