Add definitions of alpha/beta/GA to maturity levels

[psschwei]

Fixes #144

Changes

Adds definitions of alpha / beta / GA for components / features / extensions to the maturity levels doc.

Definitions are taken from: https://github.com/knative/docs/blob/main/docs/serving/configuration/feature-flags.md#lifecycle

cc @knative/technical-oversight-committee

[krsna-m]

:+1:

[evankanderson]

Is this waiting on any further approval?

/approve

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: evankanderson, psschwei

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[dprotaso]

cc @knative/technical-oversight-committee for others to review and +1

[davidhadas]

+1

[dprotaso]

/lgtm

not holding since we have quorum