Merge branch 'stable-3.1'
* stable-3.1:
Bump Bazel version to 2.2.0
Bump required Bazel version to 2.1.0
Upgrade bazlets to latest stable-2.16 to build with 2.16.16 API
Documentation: Add missing footer that links to plugin index page
Documentation: Refactor to match other plugin documentation
Add an empty tools/BUILD file explicitly for Bazel
Upgrade bazlets to latest stable-2.16
Upgrade bazlets to latest stable-2.16
Upgrade bazlets to latest stable-2.15
Upgrade bazlets to latest stable-2.14
Change-Id: I436825569e983e06b338030d135e99393925f829