Use more standard TOC marker

CONTRIBUTING.md

docs/simplesamlphp-advancedfeatures.md

docs/simplesamlphp-authproc.md

docs/simplesamlphp-changelog.md

docs/simplesamlphp-customauth.md

docs/simplesamlphp-database.md

docs/simplesamlphp-errorhandling.md

docs/simplesamlphp-googleapps.md

docs/simplesamlphp-idp.md

docs/simplesamlphp-install.md

docs/simplesamlphp-maintenance.md

docs/simplesamlphp-metadata-extensions-attributes.md

docs/simplesamlphp-metadata-extensions-rpi.md

docs/simplesamlphp-metadata-extensions-ui.md

docs/simplesamlphp-metadata-pdostoragehandler.md

docs/simplesamlphp-modules.md

docs/simplesamlphp-nostate.md

docs/simplesamlphp-reference-idp-hosted.md

docs/simplesamlphp-reference-idp-remote.md

docs/simplesamlphp-reference-sp-remote.md