Update cert-manager docs (#1926)

* updated automatic control plane rotation docs

Signed-off-by: Flynn <flynn@buoyant.io>

* fixed style recommendations

Signed-off-by: Alen Haric <aharic88@gmail.com>

* fixing linter style conflicts

Signed-off-by: Alen Haric <aharic88@gmail.com>

* WIP updates

Signed-off-by: Flynn <flynn@buoyant.io>

* Update cert-manager instructions

Signed-off-by: Flynn <flynn@buoyant.io>

* Fix lint

Signed-off-by: Flynn <flynn@buoyant.io>

* Address feedback

Signed-off-by: Flynn <flynn@buoyant.io>

* Address review feedback

Signed-off-by: Flynn <flynn@buoyant.io>

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Signed-off-by: Flynn <flynn@buoyant.io>
Signed-off-by: Alen Haric <aharic88@gmail.com>
Co-authored-by: Alen Haric <aharic88@gmail.com>

Author: kflynn