gopls: add a go:generate rule for doc generation

Change-Id: I31ade9b6eec5ff814fc803a82a182d0f92237d59
Reviewed-on: https://go-review.googlesource.com/c/tools/+/369775
Reviewed-by: Robert Findley <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: hyangah

gopls/main.go

@@ -11,6 +11,8 @@
 // for the most up-to-date documentation.
 package main // import "golang.org/x/tools/gopls"
 
+//go:generate go run doc/generate.go
+
 import (
 	"context"
 	"golang.org/x/tools/internal/analysisinternal"