fix: Preview build doc corrections (#4053)

## Explanation

The preview build documentation wrongly implied that the steps for using
a local or preview build were different for MetaMask team members versus
independent contributors. The steps are exactly the same.

Additionally, the examples that use npm have been removed. The override
syntax listed was not correct (npm doesn't use the same syntax for the
version specifier), and the other instructions all reference Yarn anyway
so the document already doesn't make much sense for a reader only
familiar with npm.

## References

N/A

## Changelog

None

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've highlighted breaking changes using the "BREAKING" category
above as appropriate

docs/contributing.md

@@ -72,17 +72,11 @@ If you're developing your project locally and want to test changes to a package,
 
       > **Example:**
       >
-      > - If you're a member of MetaMask, your project uses Yarn, `@metamask/controller-utils` is listed in dependencies at `^1.1.4`, and your clone of the `core` repo is at the same level as your project, add the following to `resolutions`:
+      > - If your project uses Yarn, `@metamask/controller-utils` is listed in dependencies at `^1.1.4`, and your clone of the `core` repo is at the same level as your project, add the following to `resolutions`:
       >
       >   ```
       >   "@metamask/controller-utils@^1.1.4": "file:../core/packages/controller-utils"
       >   ```
-      >
-      > - If you are an individual contributor, your project uses NPM, `@metamask/assets-controllers` is listed in dependencies at `^3.4.7`, and your fork of the `core` repo is at the same level as your project, add the following to `overrides`:
-      >
-      >   ```
-      >   "@metamask/assets-controllers@^3.4.7": "file:../core/packages/assets-controllers"
-      >   ```
 
    4. Run `yarn install`.
 
@@ -145,17 +139,11 @@ To use a preview build for a package within a project, you need to override the
 
    > **Example:**
    >
-   > - If you're a member of MetaMask, your project uses Yarn, `@metamask/controller-utils` is listed in dependencies at `^1.1.4`, and you want to use the preview version `1.2.3-preview-e2df9b4`, add the following to `resolutions`:
+   > - If your project uses Yarn, `@metamask/controller-utils` is listed in dependencies at `^1.1.4`, and you want to use the preview version `1.2.3-preview-e2df9b4`, add the following to `resolutions`:
    >
    >   ```
    >   "@metamask/controller-utils@^1.1.4": "npm:@metamask-previews/[email protected]"
    >   ```
-   >
-   > - If you are an individual contributor, your project uses NPM, `@metamask/assets-controllers` is listed in dependencies at `^3.4.7`, and you want to use the preview version `4.5.6-preview-bc2a997` published under `@foo`, add the following to `overrides`:
-   >
-   >   ```
-   >   "@metamask/assets-controllers@^3.4.7": "npm:@foo/[email protected]"
-   >   ```
 
 4. Run `yarn install`.