Document the releasing process

[dtantsur]

Essentially, take https://github.com/metal3-io/baremetal-operator/blob/main/docs/releasing.md, remove things that do not apply (references to CAPI, tags for submodules), add updating the default Ironic version once the branch is created (so that the release does not default to version: latest but instead to a specific latest version).

Requires having the actual release automation to generate release notes (also copy from BMO?).

/triage accepted

[dtantsur]

#118 is the first step

[tuminoid]

This should wait until BMO docs are updated with the new release machinery, which is happening in metal3-io/baremetal-operator#2132 for the next cycle. Otherwise we need to update docs twice.