Update automated docs pages for Hugo site launch

What does this MR do and why?

Updates templates for automated docs pages in order to work with the new Hugo website.

This includes these pages:

Page	Before	After
Deprecations	https://docs.gitlab.com/ee/update/deprecations.html	https://new.docs.gitlab.com/upstream-review-mr-ee-177193/update/deprecations/
Audit event types	https://docs.gitlab.com/ee/user/compliance/audit_event_types.html	https://new.docs.gitlab.com/upstream-review-mr-ee-177193/user/compliance/audit_event_types/
Custom role permissions	https://docs.gitlab.com/ee/user/custom_roles/abilities.html	https://new.docs.gitlab.com/upstream-review-mr-ee-177193/user/custom_roles/abilities/
Fine-grained permissions	https://docs.gitlab.com/ee/ci/jobs/fine_grained_permissions.html	https://new.docs.gitlab.com/upstream-review-mr-ee-177193/ci/jobs/fine_grained_permissions/

Changes required for Hugo:

1. Page titles move from h1s to title: front matter attributes
2. index.md files are renamed to _index.md
3. Custom elements (e.g, alert boxes, SVG icons) are now marked up as shortcodes

Do not merge until the Hugo launch.

References

• Prepare automated pages for Hugo (gitlab-org/technical-writing/docs-gitlab-com#168 - closed) • Sarah German • 17.9

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