added external/internal toc topics

Author: davidchesnut

reference/spfx/external-modules.md

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+# External modules
+
+External modules are typings provided for 3rd party libraries that the SharePoint Framework uses.
+
+| External module   | Description                                             |
+|-------------------|---------------------------------------------------------|
+| [es6-collections](es6-collections.md)     |                           |
+| [es6-promise](es6-promise.md)   |                     |
+| [whatwg-fetch](whatwg-fetch.md)   |                     |

reference/spfx/internal-modules.md

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+# Internal modules
+
+Internal modules are provided with the SharePoint Framework and provide access to functionality in both SharePoint and the SharePoint Framework.
+
+| Internal module   | Description                                             |
+|-------------------|---------------------------------------------------------|
+| [sp-webpart-base](sp-webpart-base-module.md)   | Provides access to basic web part functions.            |
+| [sp-core-library](sp-core-library-module.md)   | Provides core functions the rest of the framework relies on. |
+| [sp-component-base](sp-component-base-module.md) | Provides interface for accessing client-side component business logic. |
+| [sp-http](sp-http-module.md)           | Provides functionality for access to the SharePoint REST API.  |
+| [sp-loader](sp-loader-module.md)         | Used to load scripts and components.                    |
+| [sp-odata-types](sp-odata-types.md)         | Use for OData types.                    |
+| [sp-page-context](sp-page-context.md)         | Provides access to the page context.                    |