[improve][doc] Restructure releases docs

[tisonkun]

Move "Creating GPG key" and "Release note guide" as subpages of "Release process".

Thanks for @Anonymitaet and @momo-jun's suggestion.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[tisonkun]

The "version policy" part is to be handled by apache/pulsar#19570.

[momo-jun]

LGTM. It's a better version than the previous one.

And I think it's even better to have an overview page to:

1. Introduce basic concepts, like version policy (may not need an individual topic for this).
2. Introduce a map for different roles (release managers/maintainers) to read specific topics like releasing a Pulsar version, verifying release candidates, etc.

[tisonkun]

@momo-jun That sounds good. I'll try to apply it when I finish apache/pulsar#19570.

[Anonymitaet]

Tasks for the release can be divided to:

1. Merge patches from master
2. Prepare and verify RC
3. Test

If we can document these clearly, it's easier for different people to collaborate on publishing a new release.