Programmable transaction Docs - add applications

[kchalkias]

In the PTB docs we should better real world applications of the concept, this help the reader understand the importance:
examples include:
a) user batched txns (i.e. split merge coins and transfer, mass minting etc)
b) flash loans (buy from X, convert to A, sell to Y etc)
c) pay gas without Sui coins (ie pay via transferring to a sponsor another token type, do the swap and the sponsor will put the gas object)
d) attach to your transaction some metadata if i.e. the original function does not support it.
and other examples we can think of.