Improve the markdown output for GN's reference documentation.

This CL revises the markdown output that `gn help --markdown all`
produces, so that there is a table of contents at the beginning,
and so that every section has a fragment tag that you can link to,
e.g., reference.md#action_foreach.

R=brettw@chromium.org

Change-Id: If88f5ae6be5c502b23d46d1d2026ed55164d1e48
Reviewed-on: https://chromium-review.googlesource.com/455260
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#457122}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 20f4feafa5d6c72cb25737bd87af559e5046c96d
4 files changed
tree: 2a8243a3cced4527905ee5d6aa61b35e3af6709d
  1. tools/