Add MetabaseSettings.docsUrl(), use it for consistent docs links

bin/verify-doc-links

frontend/src/metabase/lib/settings.js

frontend/src/metabase/nav/components/ProfileLink.jsx

frontend/src/metabase/new_query/containers/MetricSearch.jsx

...nd/src/metabase/query_builder/components/DataSelector.jsx

...query_builder/components/expressions/ExpressionWidget.jsx

.../query_builder/components/template_tags/TagEditorHelp.jsx

.../metabase/reference/guide/GettingStartedGuideEditForm.jsx

frontend/src/metabase/reference/metrics/MetricList.jsx

frontend/src/metabase/reference/segments/SegmentList.jsx

frontend/src/metabase/setup/components/PreferencesStep.jsx

frontend/src/metabase/setup/components/Setup.jsx

frontend/src/metabase/tutorial/QueryBuilderTutorial.jsx