[Documentaion] Document the addition of `count` alongside `sum` in the `SMST`

[Olshansk]

Objective

Make all the docs reflect the addition of the cryptographically secure non-empty leaf counter.

Origin Document

Goals

• Explain why & how a cryptographically secure non-empty leaf counter was added

Deliverables

• Document the changes in [Leaf Counter] Adding a cryptographically secure non-empty leaf counter #46

Non-goals / Non-deliverables

• Modify any code

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Creator: @Olshansk
Co-Owners: @h5law