mdbook: Add links to source code at top page
mdbook: Add links to source code at top page

BUG=none
TEST=none

Change-Id: Ife7f1d7c8d5885f72e9b489e62735e8b236c1750
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3245056
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm