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

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;

```rust
/// 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),
/// Can't be accessed directly but are kept in the derived wasm bytes.
Template,
}
```