specs: increase number of reserved bytes to two

[rootulp]

Problem

https://github.com/celestiaorg/celestia-app/pull/850 increased the number of reserved bytes from 1 to 2: https://github.com/celestiaorg/celestia-app/blob/45353941dc365630b42b096fb587044ab6fa36f2/pkg/appconsts/appconsts.go#L28-L30

so the specs are now inaccurate https://github.com/celestiaorg/celestia-app/blob/2cc65da4e2663b4ba94e8be7861fe81f2be5d09c/specs/src/specs/consensus.md?plain=1#L58

Proposal

1. Update the constant from 1 to 2
2. Update the diagrams and text in the Share section

[rootulp]

This is blocked on https://github.com/celestiaorg/celestia-app/pull/856

[evan-forbes]

closed by #939