Update image URLs in dashboard Katacoda tutorial

Katacoda supports linking to images in the assets folder. Update our
existing links to use this approach instead of using the GitHub 'raw'
URLs as this gives us greater portability for the tutorials and ensures
the correct version of the images is loaded even if we stop loading content
from the `master` branch in future.

tutorials/katacoda/tekton-dashboard/step1.md

@@ -77,4 +77,4 @@ https://[[HOST_SUBDOMAIN]]-80-[[KATACODA_HOST]].environments.katacoda.com/.
 
 It might take a minute for the ingress and Katacoda to get set up.
 
-![Dashboard homepage screenshot](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/dashboard-homepage.png)
+![Dashboard homepage screenshot](./assets/dashboard-homepage.png)

tutorials/katacoda/tekton-dashboard/step2.md

@@ -26,7 +26,7 @@ Service Account `tekton-dashboard`
 
 The form should look like the following:
 
-![Import Tekton resources screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/import-tekton-resources.png)
+![Import Tekton resources screenshot.](./assets/import-tekton-resources.png)
 
 Click the `Import and Apply` button.
 
@@ -37,8 +37,8 @@ The Dashboard creates a PipelineRun to import the specified Tekton resources.
 Click on the `View status of this run` link at the bottom of the page to view
 the status of importing the Tekton resources for MyApp.
 
-![View status of importing Tekton resources screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/view-status-of-pipeline0.png)
+![View status of importing Tekton resources screenshot.](./assets/view-status-of-pipeline0.png)
 
 The Tekton resources have been imported when the PipelineRun has completed.
 
-![Import Tekton resources PipelineRun logs screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/import-pipelinerun-logs.png)
+![Import Tekton resources PipelineRun logs screenshot.](./assets/import-pipelinerun-logs.png)

tutorials/katacoda/tekton-dashboard/step3.md

@@ -11,7 +11,7 @@ Or navigate to the `default` Namespace's PipelineResources page in the Dashboard
 
 *Make sure the `default` Namespace is selected in the Namespace dropdown.*
 
-![Navigate to PipelineResources page screenshot](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/pipeline-resources-page.png)
+![Navigate to PipelineResources page screenshot](./assets/pipeline-resources-page.png)
 
 ## Create the PipelineResource
 
@@ -33,8 +33,8 @@ Revision: `master`{{copy}}
 
 The form should look like the following:
 
-![Create PipelineResource form screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/create-pipeline-resource-form.png)
+![Create PipelineResource form screenshot.](./assets/create-pipeline-resource-form.png)
 
 Click the `Create` button to create the PipelineResource.
 
-![Created PipelineResource screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/created-pipeline-resource.png)
\ No newline at end of file
+![Created PipelineResource screenshot.](./assets/created-pipeline-resource.png)

tutorials/katacoda/tekton-dashboard/step4.md

@@ -8,7 +8,7 @@ https://[[HOST_SUBDOMAIN]]-80-[[KATACODA_HOST]].environments.katacoda.com/#/name
 
 Or navigate to the `default` Namespace's PipelineRuns page.
 
-![Navigate to PipelineRuns page screenshot](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/pipeline-runs-page.png)
+![Navigate to PipelineRuns page screenshot](./assets/pipeline-runs-page.png)
 
 ## Create the PipelineRun
 
@@ -29,8 +29,8 @@ Leave the default values for the rest of the fields.
 
 The form should look like the following:
 
-![Create PipelineRun form screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/create-pipeline-run-form.png)
+![Create PipelineRun form screenshot.](./assets/create-pipeline-run-form.png)
 
 Click the `Create` button to create the PipelineRun.
 
-![Created PipelineRun screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/created-pipeline-run.png)
+![Created PipelineRun screenshot.](./assets/created-pipeline-run.png)

tutorials/katacoda/tekton-dashboard/step5.md

@@ -6,15 +6,15 @@ Then, we will open MyApp.
 View the PipelineRun logs for the MyApp PipelineRun by clicking on the link in
 the creation notification at the top of the page.
 
-![PipelineRun creation notification screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/pipeline-run-created-notification.png)
+![PipelineRun creation notification screenshot.](./assets/pipeline-run-created-notification.png)
 
 Or select the PipelineRun from the list of PipelineRuns.
 
-![View the running PipelineRun logs for MyApp screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/pipeline-run-running.png)
+![View the running PipelineRun logs for MyApp screenshot.](./assets/pipeline-run-running.png)
 
 Verify both the `build` and `deploy` tasks have passed.
 
-![View the completed PipelineRun logs for MyApp screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/pipeline-run-completed.png)
+![View the completed PipelineRun logs for MyApp screenshot.](./assets/pipeline-run-completed.png)
 
 ## Open the Deployed App
 
@@ -22,4 +22,4 @@ MyApp will be running on port `3000`. Click on the following link to open the
 app:
 https://[[HOST_SUBDOMAIN]]-3000-[[KATACODA_HOST]].environments.katacoda.com/
 
-![View MyApp screenshot.](https://raw.githubusercontent.com/ncskier/katacoda/master/tekton-dashboard/images/view-myapp.png)
\ No newline at end of file
+![View MyApp screenshot.](./assets/view-myapp.png)