Remove (s) as suggested by style guide

doc/ci/services/index.md

doc/development/adding_database_indexes.md

doc/development/code_intelligence/index.md

doc/development/code_review.md

doc/development/contributing/design.md

doc/development/contributing/merge_request_workflow.md

doc/development/database/avoiding_downtime_in_migrations.md

doc/development/database/loose_foreign_keys.md

doc/development/database/table_partitioning.md

doc/development/fe_guide/graphql.md