MM-69100 - Add team membership ABAC documentation and update channel ABAC pages for team support

[pvev]

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Summary

Documents the Team Membership ABAC feature (PR #37054 / MM-69100) and updates the existing ABAC docs to reflect that policies can now be assigned to teams, not just channels.

What's included

New page — abac-team-membership.rst:

• Advisory (public) vs strict (private) enforcement model, keyed on allow_open_invite
• Prerequisites + feature-flag behavior matrix (what changes when
  EnableAttributeBasedAccessControl / TeamMembershipAccessControl are off)
• System Admin config: policy assignment, custom rules, both auto-add checkboxes, sync
  footer, Membership sync jobs Teams tab
• Team Admin config: Team Membership tab, system-policy banner, custom rules, auto-add,
  test matching users, save confirmation, self-exclusion block, sync footer
• End-user surfaces: Browse Teams (hidden / Recommended chip), Invite modal, Add Members
  admin flow, Team Members modal, removal/auto-add DMs
• Policy inheritance, sync execution order, mass-removal guardrail, group-sync mutual
  exclusivity
• Troubleshooting FAQ

Access tab UI change (all deployments): Prominently documents that the "Allow any
user to join" checkbox is permanently replaced by Public/Private selection cards on
every team, regardless of ABAC or license. The cards control the single
allow_open_invite field (same field the checkbox did); type is intentionally left
untouched.

Updated pages:

• attribute-based-access-control.rst — toctree entry, team policy type, deduped roles
  lists
• abac-system-wide-policies.rst — "Assign policies to teams" section; delete now
  requires 0 channels and 0 teams
• abac-team-channel-policies.rst — "Membership Policies" tab renamed to "Channel
  Membership"; disambiguation note vs. the new Team Membership tab

Ticket Link

https://mattermost.atlassian.net/browse/MM-69100

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 75b10e9