docs: update read_file docs & clarify package restrictions (#1540)

## Description:
This pull request clarifies 2 things in the docs:
- the returned type from `read_file` function in the Starlark reference
- clarifies that resources being used in a Starlark script must
themselves be a part of a Kurtosis package (i.e. a starlark script can
only read or upload or import resources/starlark scripts/files that are
part of other packages)

## Is this change user facing?
YES

## References (if applicable):
Fixes #1483

docs/docs/concepts-reference/locators.md

@@ -36,9 +36,8 @@ github.com/kurtosis-tech/kurtosis/starlark/test.star
 Only locators pointing to public GitHub repositories are currently allowed.
 :::
 
-### Package Restriction
-Any Starlark script that wishes to use external resources must be
-a part of a [Kurtosis package][packages].
+### Important Package Restriction
+If your Starlark script relies on local resources, such as files or packages available on your filesystem, then those resources *must* be part of a [Kurtosis package][packages]. 
 
 For example, suppose we had a [Kurtosis package][packages] like so:
 
@@ -60,16 +59,17 @@ with a `kurtosis.yml` file like so:
 name: github.com/package-author/package-repo/my-package
 ```
 
-The `main.star` file would import the `random-script.star` from the `helpers` subdirectory of `my-package` like so:
+In your `main.star` file, you would be able to import the `random-script.star` from the `helpers` subdirectory of `my-package` like so:
 
 ```python
+# Valid
 helpers = import_module("github.com/package-author/package-repo/my-package/helpers/random-script.star")
 ```
 
-The import statement below will not succeed, because `main.star` cannot import from non-packages.
-(see [how import works][how-do-kurtosis-imports-work-explanation] for more information)
+However, if you try to import `package-repo/not-a-package/random-script.star`, then it will not work because `package-repo/not-a-package/random-script.star` is not part of a package. In essence, the import statement below will not succeed, because `main.star` cannot import from non-packages (see [how import works][how-do-kurtosis-imports-work-explanation] for more information):
 
 ```python
+# Invalid
 helpers = import_module("github.com/package-author/package-repo/not-a-package/random-script.star")
 ```
 

docs/docs/starlark-reference/plan.md

@@ -602,7 +602,7 @@ The return value is a [future reference][future-references-reference] to the nam
 upload_files
 ------------
 
-`upload_files` instruction packages the files specified by the [locator][locators-reference] into a [files artifact][files-artifacts-reference] that gets stored inside the enclave. This is particularly useful when a static file needs to be loaded to a service container.
+The `upload_files` instruction packages the files specified by the [locator][locators-reference] into a [files artifact][files-artifacts-reference] that gets stored inside the enclave. This is particularly useful when a static file needs to be loaded to a service container. 
 
 ```python
 artifact_name = plan.upload_files(

docs/docs/starlark-reference/read-file.md

@@ -3,7 +3,7 @@ title: read_file
 sidebar_label: read_file
 ---
 
-The `read_file` function reads the contents of a file specified by the given [locator][locators-reference], and requires that the Starlark script is part of a [package][packages-reference]. `read_file` executes [at interpretation time][multi-phase-runs-reference] so the file contents won't be displayed in the preview.
+The `read_file` function reads the contents of a file specified by the given [locator][locators-reference] and executes [at interpretation time][multi-phase-runs-reference]. This instruction returns the content of the file in a string type. Please note that the files being read from must themselves be part of a Kurtosis package, as explained [here](../concepts-reference/locators.md#important-package-restriction).
 
 ```python
 read_file(