internal/api: render markdown documentation

Change-Id: I8adf1e62d19cf592cc8204264d99953e12da5843
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/760260
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ethan Lee <ethanalee@google.com>

3 files changed, 117 insertions, 0 deletions

diff --git a/internal/api/testdata/markdown.golden b/internal/api/testdata/markdown.golden
new file mode 100644
index 00000000..205546d0
--- /dev/null
+++ b/internal/api/testdata/markdown.golden
@@ -0,0 +1,105 @@
+# package pkg
+
+Package pkg has every form of declaration.
+
+### Heading {#hdr-Heading}
+
+Search [Google](https://google.com) for details.
+
+### Links {#hdr-Links}
+
+  - pkgsite repo, [https://go.googlesource.com/pkgsite](https://go.googlesource.com/pkgsite)
+  - Play with Go, [https://play-with-go.dev](https://play-with-go.dev)
+
+## Constants
+
+```
+const (
+	X = 1
+	Y = 2
+)
+```
+Several constants.
+
+```
+const C = 1
+```
+C is a shorthand for 1.
+
+## Variables
+
+```
+var V = 2
+```
+V is a variable.
+
+## Functions
+
+```
+func Add(x int) int
+```
+Add adds 1 to x.
+
+```
+func F()
+```
+F is a function.
+
+## Types
+
+```
+type A int
+```
+
+```
+type B bool
+```
+
+```
+type I1 interface {
+	M1()
+}
+```
+I1 is an interface.
+
+```
+type I2 interface {
+	I1
+	M2()
+}
+```
+
+```
+type S1 struct {
+	F int // field
+}
+```
+S1 is a struct.
+
+```
+type S2 struct {
+	S1
+	G int
+}
+```
+S2 is another struct.
+
+```
+type T int
+```
+T is a type.
+
+```
+const CT T = 3
+```
+CT is a typed constant. They appear after their type.
+
+```
+func TF() T
+```
+TF is a constructor for T.
+
+```
+func (T) M()
+```
+M is a method of T. BUG(xxx): this verifies that notes are rendered.
diff --git a/internal/api/testdata/pkg.go b/internal/api/testdata/pkg.go
index b9650bb8..6d62abad 100644
--- a/internal/api/testdata/pkg.go
+++ b/internal/api/testdata/pkg.go
@@ -4,10 +4,16 @@
 
 // Package pkg has every form of declaration.
 //
+// # Heading
+//
+// Search [Google] for details.
+//
 // # Links
 //
 //   - pkgsite repo, https://go.googlesource.com/pkgsite
 //   - Play with Go, https://play-with-go.dev
+//
+// [Google]: https://google.com
 package pkg
 
 // C is a shorthand for 1.
diff --git a/internal/api/testdata/text.golden b/internal/api/testdata/text.golden
index 6497759c..75998ec5 100644
--- a/internal/api/testdata/text.golden
+++ b/internal/api/testdata/text.golden
@@ -2,11 +2,17 @@ package pkg
 
 Package pkg has every form of declaration.
 
+# Heading
+
+Search Google for details.
+
 # Links
 
   - pkgsite repo, https://go.googlesource.com/pkgsite
   - Play with Go, https://play-with-go.dev
 
+[Google]: https://google.com
+
 CONSTANTS
 
 const (