docs: correct MBR exemptions for empty balance records and special addresses

Docs: Add Applications concepts page and examples

Docs: Add Algorand Client concepts page and examples

fnet5 is the protocol AF's FNet runs. It extends v41 with the parameter
changes go-algorand released as ConsensusV42, restated rather than derived
from v42, so fnet5 stays pinned to the values FNet runs with.

The PR workflow pins CI_E2E_FILENAME to a commit hash, so its block data
is frozen and cannot surface encoding changes in go-algorand. New block or
transaction fields are fatal rather than ignorable, because the SDK decodes
with ErrorIfNoField set, so an unknown map key aborts block import. The
unit tests cannot catch it either: they encode and decode with the same SDK
structs, which round-trips regardless of what algod puts on the wire.

Sync: Handle load, discount, pqsig

Pqsig support