This is the first PR adding vendor-specific documentation.

- Refactor installation docs to make it simpler with tabs for generic
  instructions, Google cloud, and Openshift. This is a multi-PR change.
  The actual installation doc is created on the Pipelines repo and
  rendered on the website.

- Update the build and push how-to with instructions to authenticate to
  Google Artifact Registry.

- Create a new shortcode to commenting out text in the markdown file.

Fixing a small typo which breaks the How-to Guide if followed blindly/copy-pasted.
The documentation refers to `pipeline,yaml` in line 49, but I believe it should be referring to `pipeline.yaml` ( using a punctuation instead of a comma ).

To avoid having to delete and reapply a Pipelinerun when developing your
code, this PR switches to `generateName`, which adds a suffix to have a
unique name each time.

Additionally, this PR also fixes a problem with the latest version of
the `git-clone` Task in the Git clone tutorial.

Recently, the git-clone task was updated to version 0.8 which is not
backwards compatible in that it is run as a non-root user which does not
have access to volumes mounted via workspaces by default. To accomodate
this change, the users need to update their PipelineRuns and TaskRuns
with `podTemplate.securityContext.fsGroup = 65532` to fix the
permissions on mounted volumes.

This commit makes this configuration change in the how-to guide.

Fix typographical errors in the code samples.

Correct mis-typed name of desired `pipelineRef`.

A tutorial that shows how to create a Tekton Pipeline to build and push
an image with Kaniko.