Add Juno halving calculation to the Economic overview section

[kopeboy]

Refer to the commit messages for the list of proposed changes.

The end of phase calculation was provided by @CosmosCamel and is being included in the documentation as a self-service replacement of the recently sunsetted junohalving.io website.

[kopeboy]

Regarding the ~~$JUNO~~ JUNO: I suggest we remove the $ sign prefix because:

1. it's not connected to any UI filter (ie. producing any link or popup, like for example it does on X.com),
2. for coherence because it's not used on ATOM or ujuno (which are denoms just like JUNO) anywhere in the docs,
3. because it may cause confusion for readers imho: $ commonly means USD or some other & specific local fiat currencies, which have nothing to do with JUNO (ie. "$" does not mean "currency" in general).