docs: restructure user.md
docs: restructure user.md

Change-Id: Ife0b8c621f0039d9e937873ff45282676ab20f3d

bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration