Adding release cut onboarding handbook

[mbianchidev]

This PR is the result of merging my onboarding notes and a HackMD provided during my first release cut.

What type of PR is this:

/kind documentation
/priority important-longterm

What this PR does / why we need it:

This is part of a bigger plan to document release engineering processes meanwhile I get onboarded in the team.
The doc is meant to be scoped to just cover a release cut and provide a step by step guide to perform it.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

CC: @jimangel

[k8s-ci-robot]

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[mbianchidev]

/hold until this gets more eyes on it and some consensus

[puerco]

Thanks for the update to the docs Matteo, updates to docs are always needed and welcome! Note, however, that this is duplicating the image promotion docs and the branch manager handbook (check the Releases Management section). So I think it's better if we update the kpromo docs and the handbook based on your notes.

[mbianchidev]

Thanks for the update to the docs Matteo, updates to docs are always needed and welcome! Note, however, that this is duplicating the image promotion docs and the branch manager handbook (check the Releases Management section). So I think it's better if we update the kpromo docs and the handbook based on your notes.

Thanks for the review! @puerco

Note: As I am currently onboarding I surely lack the context and historical reasons behind the choice of having those as fragmented docs.

I found (and still find) them to be a tiny bit difficult to navigate as a new contributor and more focused on giving a general overview on the process rather than a straightforward step by step guide to actually perform a release cut.
Now, I also agree that some of the content is lacking on this PR and could definitely be integrated with the existing content.
I don't think that operating the other way around would be beneficial for new contributors.

The option I find viable are the following:

1. Integrating and substituting

Integrate the existing content in this new handbook so that it can become a more complete guide focused just on release cuts (including a step by step guide) - and linking it back in the branch manager handbook as follows:

## Releases Management

[...]

### Release cut complete guide

You can find a step by step buide [here](/handbooks/k8s-release-cut.md)

I have no opinion on temporarily duplicating some of the content and initially keep it in both places, but in the long term I think it would be better to have smaller focused and step by step guides and kind of "breaking down" the current branch manager handbook in smaller, more digestible chunks.

2. Just linking

Linking this new guide as it is in the references section of the branch manager handbook and remove legacy links as follows:

## References
- [Release Tools Documentation](https://github.com/kubernetes/release/blob/master/README.md)
- [Release cut step by step guide](/handbooks/k8s-release-cut.md)

Let me know what you think!