internal/api: render text documentation

Preliminary attempt to render documentation as text.

The output doesn't match either pkgsite or go doc.
It's a simplification of the latter.
The go doc command turns out to be surprisingly complicated,
so it's best to start simple.

Change-Id: I7b5a6bf36b1892afb30c212309dd646e3cf8b06a
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/757501
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Ethan Lee <ethanalee@google.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 78 insertions, 0 deletions

diff --git a/internal/api/testdata/pkg.go b/internal/api/testdata/pkg.go
new file mode 100644
index 00000000..b9650bb8
--- /dev/null
+++ b/internal/api/testdata/pkg.go
@@ -0,0 +1,78 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package pkg has every form of declaration.
+//
+// # Links
+//
+//   - pkgsite repo, https://go.googlesource.com/pkgsite
+//   - Play with Go, https://play-with-go.dev
+package pkg
+
+// C is a shorthand for 1.
+const C = 1
+
+// No exported name; should not appear.
+const (
+	a = 1
+	b = 2
+	c = 3
+)
+
+// V is a variable.
+var V = 2
+
+// F is a function.
+func F() {}
+
+// Several constants.
+const (
+	X = 1
+	Y = 2
+)
+
+// CT is a typed constant.
+// They appear after their type.
+const CT T = 3
+
+// TF is a constructor for T.
+func TF() T { return T(0) }
+
+// M is a method of T.
+// BUG(xxx): this verifies that notes are rendered.
+func (T) M() {}
+
+// T is a type.
+type T int
+
+// S1 is a struct.
+type S1 struct {
+	F int // field
+}
+
+// S2 is another struct.
+type S2 struct {
+	S1
+	G int
+}
+
+// I1 is an interface.
+type I1 interface {
+	M1()
+}
+
+type I2 interface {
+	I1
+	M2()
+}
+
+type (
+	A int
+	B bool
+)
+
+// Add adds 1 to x.
+func Add(x int) int {
+	return x + 1
+}