_content/doc: add a table of contents to comment.md

comment.md is longer than pgo.md and build-cover.md, both of which have
tables of contents.

Change-Id: Ie73e2405d0d1ac9c245544066250f62f244cbead
Reviewed-on: https://go-review.googlesource.com/c/website/+/664598
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: bitlux

_content/doc/comment.md

@@ -4,6 +4,17 @@ layout: article
 date: 2022-06-01T00:00:00Z
 ---
 
+Table of Contents:
+
+ [Packages](#package)\
+ [Commands](#cmd)\
+ [Types](#type)\
+ [Funcs](#func)\
+ [Consts](#const)\
+ [Vars](#var)\
+ [Syntax](#syntax)\
+ [Common mistakes and pitfalls](#mistakes)
+
 “Doc comments” are comments that appear immediately before top-level package,
 const, func, type, and var declarations with no intervening newlines.
 Every exported (capitalized) name should have a doc comment.