Add docs for new admin APIs - #88

Running Vale across on the following files affected by this PR:

- account/branding.mdx
- admin-api/endpoints/attributes/get-an-attribute.mdx
- admin-api/endpoints/leaderboards/get-a-leaderboard.mdx
- admin-api/endpoints/tenants/get-a-tenant.mdx
- es/account/branding.mdx
- es/admin-api/endpoints/attributes/get-an-attribute.mdx
- es/admin-api/endpoints/leaderboards/get-a-leaderboard.mdx
- es/admin-api/endpoints/tenants/get-a-tenant.mdx
- es/features/achievements.mdx
- es/features/emails.mdx
- es/features/streaks.mdx
- es/features/users.mdx
- es/platform/experimentation.mdx
- features/achievements.mdx
- features/emails.mdx
- features/streaks.mdx
- features/users.mdx
- platform/experimentation.mdx
- snippets/user-streak-preferences-response-block.mdx
- admin-api/endpoints/achievements/create-achievements.mdx
- admin-api/endpoints/achievements/delete-achievements.mdx
- admin-api/endpoints/achievements/get-an-achievement.mdx
- admin-api/endpoints/achievements/list-achievements.mdx
- admin-api/endpoints/achievements/update-achievements.mdx
- admin-api/endpoints/settings/get-organization-settings.mdx
- admin-api/endpoints/settings/update-organization-settings.mdx
- admin-api/endpoints/streaks/get-streak-settings.mdx
- admin-api/endpoints/streaks/update-streak-settings.mdx
- es/admin-api/endpoints/achievements/create-achievements.mdx
- es/admin-api/endpoints/achievements/delete-achievements.mdx
- es/admin-api/endpoints/achievements/get-an-achievement.mdx
- es/admin-api/endpoints/achievements/list-achievements.mdx
- es/admin-api/endpoints/achievements/update-achievements.mdx
- es/admin-api/endpoints/settings/get-organization-settings.mdx
- es/admin-api/endpoints/settings/update-organization-settings.mdx
- es/admin-api/endpoints/streaks/get-streak-settings.mdx
- es/admin-api/endpoints/streaks/update-streak-settings.mdx

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