docs(MAINTAINERS_GUIDELINES.md): core maintainers need to join with `…

[leogr]

What type of PR is this?

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

/kind bug

/kind cleanup

/kind documentation

/kind feature

/kind incubating

/kind officialsupport

Any specific area of the project related to this PR?

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

/area governance

/area proposals

/area utils

What this PR does / why we need it:

As noticed by @incertum :pray: , there was a missing step in the onboarding documentation. Core maintainers also need to join the core-maintainers team to properly receive a notification when mentioned.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jasondellaluce, leogr

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