Add a Liquid template for Github PR links (#2380)

_includes/github-pr.md

@@ -0,0 +1,12 @@
+{% comment %}
+  Generates a Github PR URL from three parameters
+
+  Usage:
+    {% include github-pr.md org=docker repo=docker pr=12345 %}
+
+    If you omit the org or repo, they default to docker.
+    If you omit the pr, it defaults to NULL.
+
+  Output:
+    [#12345](https://github.com/docker/docker/pull/12345)
+{% endcomment %}{% assign org = include.org | default: "docker" %}{% assign repo = include.repo | default: "docker" %}{% assign pr = include.pr | default: NULL %}{% assign github-url="https://github.com" %}{% capture pr-link %}[#{{ pr }}]({{ github-url }}/{{ org }}/{{ repo }}/pull/{{ pr }}){% endcapture %}{{ pr-link | strip_newlines }}

test.md

@@ -406,3 +406,15 @@ authorizedkeys:
 > | Row 2 column 1 | Row 2 column 2 |
 >
 > And another sentence to top it all off.
+
+## Linking
+
+- [a normal link](https://docker.com/)
+
+- <a href="https://docker.com/">an HTML link</a>
+
+- A link to a Github PR in `docker/docker`: {% include github-pr.md pr=28199 %}
+
+- A link to a Github PR in `docker/docker.github.io`: {% include github-pr.md repo=docker.github.io pr=9999 %}
+
+(you can also specify `org=foo` to use a Github organization other than Docker).