First compilation of krel documentation

[puerco]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

This is the firs batch of the krel documentation. Most of the subcommand docs are derived from krels help output but the idea is to have a first version of the docs from where we can improve them.

ref: #846

Special notes for your reviewer:

This is the first version of the docs. As noted above, I wrote this first batch of documents following the --help output of each subcommand. This bunch of file provides the structure to build a full featured set of documents.

If you or someone you know can provide insight to add more details, caveats, known issues or other important information please let me know.

Does this PR introduce a user-facing change?

First batch of krel usage documents. Main README.md and a doc for each subcommand.

[k8s-ci-robot]

@puerco: Adding label: do-not-merge/blocked-paths because PR changes a protected file.

Reasons for blocking this PR:

[Changes to certain release tools can affect our ability to test, build, and release Kubernetes. This PR must be explicitly approved by SIG Release repo admins.]

• docs/krel/gcbmgr.md

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[puerco]

/assign @tpepper

[saschagrunert]

Just some nits, generally LGTM

[puerco]

Ready !

[justaugustus]

Great work, @puerco!
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justaugustus, puerco

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