Redo the `about` Documentation page

We fix the dangling links, add UI screenshots, and link to the
REST API.

Change-Id: I2884b1b4c6048eab3114cceeb3a0b5ac7f1e08c2

src/main/resources/Documentation/about.md

diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index 19d67ec..4abe78f 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md
@@ -1,5 +1,18 @@
-The @PLUGIN@ plugin detects Zuul [cross repository dependencies] (CRD) in
-commit messages.
+The @PLUGIN@ plugin detects Zuul [cross repository dependencies] (CRD) in
+commit messages and displays them on the Gerrit UI as "Depends on" and "Needed
+by" sections in the related changes section.
 
-[cross repository dependencies]: http://docs.openstack.org/infra/zuul/gating.html#cross-repository-dependencies
-[Cycles]: http://docs.openstack.org/infra/zuul/gating.html#cycles
+![user interface overview](images/overview.png)
+
+[Dependency cycles] (although currently only direct dependencies are considered) get highlighted
+
+![cycles screenshot](images/cycles.png)
+
+And dependencies that are not available on the server get highlighted too.
+
+![missing dependencies screenshot](images/missing.png)
+
+The cross repository dependencies are also exposed through the [REST API](rest-api-changes.html).
+
+[cross repository dependencies]: https://zuul-ci.org/docs/zuul/discussion/gating.html#cross-project-dependencies
+[Dependency cycles]: https://zuul-ci.org/docs/zuul/discussion/gating.html#cycles

src/main/resources/Documentation/images/cycles.png

diff --git a/src/main/resources/Documentation/images/cycles.png b/src/main/resources/Documentation/images/cycles.png
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src/main/resources/Documentation/images/missing.png

diff --git a/src/main/resources/Documentation/images/missing.png b/src/main/resources/Documentation/images/missing.png
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src/main/resources/Documentation/images/overview.png

diff --git a/src/main/resources/Documentation/images/overview.png b/src/main/resources/Documentation/images/overview.png
new file mode 100644
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