Add linkcheck, fix typos and expand.

doc/src/rust/packages/create.md

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+Create a Custom Package
+======================
+
+{{#include ../../links.md}}
+
+Sometimes specific functionalities are needed, so custom packages can be created.
+
+The macro `rhai::def_package!` is used to create a new custom package.
+
+
+Macro Parameters
+---------------
+
+`def_package!(root:package_name:description, variable, block)`
+
+* `root` - root namespace, usually `"rhai"`.
+
+* `package_name` - name of the package, usually ending in `Package`.
+
+* `description` - doc comment for the package.
+
+* `variable` - a variable name holding a reference to the [module] that is to form the package.
+
+* `block` - a code block that initializes the package.
+
+```rust
+// Import necessary types and traits.
+use rhai::{
+    def_package,
+    packages::Package,
+    packages::{ArithmeticPackage, BasicArrayPackage, BasicMapPackage, LogicPackage}
+};
+
+// Define the package 'MyPackage'.
+def_package!(rhai:MyPackage:"My own personal super package", module, {
+    // Aggregate existing packages simply by calling 'init' on each.
+    ArithmeticPackage::init(module);
+    LogicPackage::init(module);
+    BasicArrayPackage::init(module);
+    BasicMapPackage::init(module);
+
+    // Register additional Rust functions using the standard 'set_fn_XXX' module API.
+    module.set_fn_1("foo", |s: ImmutableString| {
+        Ok(foo(s.into_owned()))
+    });
+});
+```