Fix: Consumer groups docs

[lena-larionova]

Description

Addressing feedback we’ve received on consumer groups docs, where some of the docs are outdated (kong manager), one is missing from the nav but findable via search (Konnect), and links for how-to guides go to the Rate Limiting plugin, which is only one of the plugins that supports consumer groups.

• The Kong Manager topic is very short and doesn't need to exist, but doesn't hurt to leave it for now
• The Konnect topic is a condensed version of upcoming how-to guides
• As part of this, splitting out the Personal Access Token info into its own doc, as we link to it a bunch (I needed to from the consumer groups doc), but its buried in a decK page.

Fixes #6465 and https://konghq.atlassian.net/browse/DOCU-3593

Testing instructions

Preview link:
https://deploy-preview-8257--kongdocs.netlify.app/gateway/latest/key-concepts/consumer-groups/
https://deploy-preview-8257--kongdocs.netlify.app/gateway/latest/kong-manager/consumer-groups/
https://deploy-preview-8257--kongdocs.netlify.app/konnect/gateway-manager/configuration/consumer-groups/
https://deploy-preview-8257--kongdocs.netlify.app/konnect/org-management/access-tokens/

Checklist

• Review label added
• Conditional version tags added, if applicable.

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[cloudjumpercat]

Left a couple suggestions, but I'll approve so you aren't blocked!