Document the `factory` pattern and `Template` entry point.

[devendran-m]

Document how to use the new Casper 2.0 and the useful factory pattern, which the Template entry point access variant in the below enum pertains to;

/// Enum describing the possible access control options for a contract entry
/// point (method).
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[cfg_attr(feature = "datasize", derive(DataSize))]
#[cfg_attr(feature = "json-schema", derive(JsonSchema))]
pub enum EntryPointAccess {
    /// Anyone can call this method (no access controls).
    Public,
    /// Only users from the listed groups may call this method. Note: if the
    /// list is empty then this method is not callable from outside the
    /// contract.
    Groups(Vec<Group>),
    /// Can't be accessed directly but are kept in the derived wasm bytes.
    Template,
}