docs(CONTRIBUTING): refine rules maturity framework

[incertum]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind feature

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

Any specific area of the project related to this PR?

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/area rules

/area registry

/area build

/area documentation

Proposed rule maturity level

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/area maturity-stable

/area maturity-incubating

/area maturity-sandbox

/area maturity-deprecated

What this PR does / why we need it:

Refine rules maturity framework, see https://github.com/falcosecurity/rules/issues/126#issuecomment-1696025071. @leogr

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[poiana]

LGTM label has been added.

Git tree hash: 3b491dc53140aff78aaec9baa363c2e1d135d5d9

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: darryk10, incertum, leogr

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/falcosecurity/rules/blob/main/OWNERS)~~ [incertum,leogr] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment