Updated handbook URLs

doc/administration/encrypted_configuration.md

doc/administration/file_hooks.md

doc/administration/get_started.md

doc/administration/index.md

doc/administration/integration/plantuml.md

doc/administration/invalidate_markdown_cache.md

doc/administration/issue_closing_pattern.md

doc/administration/lfs/index.md

doc/administration/snippets/index.md

doc/administration/static_objects_external_storage.md

doc/administration/whats-new.md

doc/api/access_requests.md

doc/api/branches.md

doc/api/group_wikis.md

doc/api/keys.md

doc/api/merge_request_approvals.md

doc/api/merge_request_context_commits.md

doc/api/packages/npm.md

doc/api/packages/nuget.md

doc/api/project_aliases.md