PublicDashboards: Add setting to disable the feature (#78894)

* Replace feature toggle with configuration setting

* Fix permission alert

* Update documentation

* Add back feature toggle

* revert unwanted commited changes

* fix tests

* run prettier

* Update SharePublicDashboard.test.tsx

* fix linter and frontend tests

* Update api.go

* Apply docs edit from code review

Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>

* Update index.md

* Update docs/sources/setup-grafana/configure-grafana/feature-toggles/index.md

Co-authored-by: Agnès Toulet <35176601+AgnesToulet@users.noreply.github.com>

* Update docs/sources/setup-grafana/configure-grafana/_index.md

Co-authored-by: Agnès Toulet <35176601+AgnesToulet@users.noreply.github.com>

* add isPublicDashboardsEnabled + test

* fix test

* update ff description in registry

* move isPublicDashboardsEnabled

* revert getConfig() update

---------

Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>
Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

conf/sample.ini

@@ -1615,3 +1615,9 @@
 ;hidden_toggles =
 # Disable updating specific feature toggles in the feature management page
 ;read_only_toggles =
+
+#################################### Public Dashboards #####################################
+[public_dashboards]
+# Set to false to disable public dashboards
+;enabled = true
+