docs: add pages for teams, team folders and the Teams API

[miaulalala]

Why

Teams and team folders have no page in any manual. "Team folder" appears 19 times across the three manuals — activity configuration, encryption, primary storage, trashbin, automated tagging — always in passing, and never defined. "Team space" appears zero times. The Teams API has no developer documentation at all, even though ITeamResourceProvider has existed since 29.0.0 and Files, Talk and Deck implement it.

The specific gap that prompted this: you can create a team and then discover it cannot own anything. Nothing documented that, or that Nextcloud 35 addresses it with team spaces.

What this adds

One page per manual, no content mixed between them:

• user_manual/groupware/teams.rst — what a team is (a membership list, not a storage location), that it owns nothing until it has a team space, how a space is obtained, roles, and an FAQ. Includes the warning that detaching a space keeps the files while deleting it does not.
• admin_manual/configuration_files/team_folders.rst — team folders versus team spaces as a comparison table, which teams are ineligible (personal, hidden, system, backend-provided), the two circles app settings that gate provisioning (team_folder_auto_create, team_folder_default_quota, neither exposed in the admin UI), detach versus delete, links to the pages that already discuss team folders in passing, and an FAQ.
• developer_manual/digging_deeper/teams.rst — ITeamResourceProvider, ITeamManager (with the @since of each method, including getSharedWithList() gaining $resourceId in 34.0.0), TeamResource, and the ITeamFolderProvider added in 35.0.0, plus registration via registerTeamResourceProvider() and the four existing implementations.

Wired into the three toctrees.

Notes for review

• Everything is derived from the code in circles, groupfolders and lib/public/Teams, not from the UI, so the user-facing wording deserves a check against the actual interface labels — the code says "team space" while the admin surface says "Team folders".
• Who may add a team space is documented as the team owner or a server administrator, taken from the permission check on the endpoint rather than from a spec.
• The screenshot in contacts.rst is still stale and is being handled separately (see fix(user): teams are managed in the Teams app, not embedded in Contacts #15590, which fixes the claim that Teams is embedded in Contacts).
• I have not moved the existing Teams section out of contacts.rst, to keep this reviewable; the new user page and that section overlap on roles. Happy to fold one into the other in a follow-up once the placement is agreed.

[miaulalala]

/backport to stable35