Add internal link verification via `mkdocs build --strict` (#3931)

* Add internal link verification via `mkdocs build --strict`

* Fix broken links

.github/workflows/mkdocs-strict-verify.yaml

@@ -0,0 +1,50 @@
+# Copyright 2020 The Knative Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This file is automagically synced here from github.com/knative-sandbox/.github
+# repo by knobots: https://github.com/knative-sandbox/knobots and will be overwritten.
+
+name: Knative Verify
+
+on:
+  pull_request:
+    branches: [ 'mkdocs', 'release-*' ]
+
+jobs:
+
+  verify:
+    name: Verify mkdocs strict
+    strategy:
+      matrix:
+        go-version: [1.16.x]
+        platform: [ubuntu-latest]
+
+    runs-on: ${{ matrix.platform }}
+
+    env:
+      GOPATH: ${{ github.workspace }}
+
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+
+    - name: Set up Python
+      uses: actions/setup-python@v2
+
+    - name: Install requirements
+      run: python3 -m pip install -r requirements.txt
+
+    - name: Verify MkDocs
+      run: mkdocs build --strict

docs/eventing/sources/README.md

@@ -42,7 +42,7 @@ All Sources are part of the `sources` category.
 | [Apache Camel](./apache-camel-source) | N/A   | Apache Software Foundation    | Enables use of [Apache Camel](https://github.com/apache/camel) components for pushing events into Knative. Camel sources are now provided via [Kamelets](https://camel.apache.org/camel-kamelets/latest/) as part of the [Apache Camel K](https://camel.apache.org/camel-k/latest/installation/installation.html) project. |
 | [Apache CouchDB](https://github.com/knative-sandbox/eventing-couchdb/blob/main/source)                                                   | v1alpha1 | Knative    | Brings [Apache CouchDB](https://couchdb.apache.org/) messages into Knative.  |
 | [Apache Kafka](../samples/kafka)       | v1beta1  | Knative    | Brings [Apache Kafka](https://kafka.apache.org/) messages into Knative. The KafkaSource reads events from an Apache Kafka Cluster, and passes these events to a sink so that they can be consumed. See the [Kafka Source](https://github.com/knative-sandbox/eventing-kafka/blob/main/pkg/source) example for more details.  |
-| [Container Source](./containersource.md)                    | v1 | Knative    | The ContainerSource will instantiate container image(s) that can generate events until the ContainerSource is deleted. This may be used, for example, to poll an FTP server for new files or generate events at a set time interval. Given a `spec.template` with at least a container image specified, ContainerSource will keep a `Pod` running with the specified image(s). `K_SINK` (destination address) and `KE_CE_OVERRIDES` (JSON CloudEvents attributes) environment variables are injected into the running image(s). It is used by multiple other Sources as underlying infrastructure. Refer to the [Container Source](./container-source) example for more details.  |
+| [Container Source](./containersource/README.md)                    | v1 | Knative    | The ContainerSource will instantiate container image(s) that can generate events until the ContainerSource is deleted. This may be used, for example, to poll an FTP server for new files or generate events at a set time interval. Given a `spec.template` with at least a container image specified, ContainerSource will keep a `Pod` running with the specified image(s). `K_SINK` (destination address) and `KE_CE_OVERRIDES` (JSON CloudEvents attributes) environment variables are injected into the running image(s). It is used by multiple other Sources as underlying infrastructure. Refer to the [Container Source](./container-source) example for more details.  |
 | [GitHub](./github-source)            | v1alpha1 | Knative    | Registers for events of the specified types on the specified GitHub organization or repository, and brings those events into Knative. The GitHubSource fires a new event for selected [GitHub event types](https://developer.github.com/v3/activity/events/types/). See the [GitHub Source](./github-source) example for more details.  |
 | [GitLab](./gitlab-source) | v1alpha1 | Knative    | Registers for events of the specified types on the specified GitLab repository, and brings those events into Knative. The GitLabSource creates a webhooks for specified [event types](https://docs.gitlab.com/ee/user/project/integrations/webhooks.html#events), listens for incoming events, and passes them to a consumer. See the [GitLab Source](./gitlab-source) example for more details.  |
 | [Heartbeats](https://github.com/knative/eventing/tree/main/cmd/heartbeats) |  N/A | Knative    | Uses an in-memory timer to produce events at the specified interval. |

docs/getting-started/first-source.md

@@ -80,11 +80,11 @@ Create the CloudEvents Player Service:
   1. Ensure your Event Source does not contain any spaces.
   1. Click **SEND EVENT**.
 
-![CloudEvents Player Send](../images/event_sent.png)
+![CloudEvents Player Send](images/event_sent.png)
 
 
 ??? tip "Clicking the :fontawesome-solid-envelope: shows you the CloudEvent as the Broker sees it."
-    ![Event_Details](../images/event_details.png)
+    ![Event_Details](images/event_details.png)
 
 The :material-send: icon in the "Status" column implies that the event has been sent to our Broker... but where has the event gone? **Well, right now, nowhere!**