docs: adopt consistent terminology (app vs smart contracts)

[cusma]

We used "Smart Contracts" 11 times. (And "Application" probably 15 or so).

I'd prefer to leave to another PR any cleanup in this regard. We have to deal with the opening paragraph:

The AVM is a bytecode based stack interpreter that executes programs
associated with Algorand transactions. TEAL is an assembly language
syntax for specifying a program that is ultimately converted to AVM
bytecode. These programs can be used to check the parameters of the
transaction and approve the transaction as if by a signature. This use
is called a _Smart Signature_. Starting with v2, these programs may
also execute as _Smart Contracts_, which are often called
_Applications_. Contract executions are invoked with explicit
application call transactions.

if we want to eliminate "Smart Contract". "Smart Signature" is the uglier one, in my opinion, but this was one of those failed attempts to choose catchy marketing terms to name important features that never really caught on.

Originally posted by @jannotti in #188 (comment)