Docs: Remove old plugin docs, fix URLs to go to new dev portal (#75325)

* Remove old plugin docs, add redirects to new dev portal

* Restore plugin.schema.json file

* Update docs/sources/administration/plugin-management/index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* More updates

* Cleanup links

* Update docs/sources/alerting/fundamentals/evaluate-grafana-alerts.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* fix codeowners

* Change reference links to inline links to avoid 404s

* Update docs/sources/datasources/_index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* Update docs/sources/datasources/mysql/_index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* Update docs/sources/datasources/postgres/_index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* Update docs/sources/fundamentals/timeseries-dimensions/index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* Update docs/sources/fundamentals/dashboards-overview/index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* Update docs/sources/fundamentals/dashboards-overview/index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* Update docs/sources/panels-visualizations/configure-standard-options/index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* Update docs/sources/panels-visualizations/query-transform-data/transform-data/index.md

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

* Remove grafana.com from 2 URLs

* Fix 1 more reference link

* Prettier fix

---------

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

.github/CODEOWNERS

@@ -53,7 +53,6 @@
 /docs/sources/upgrade-guide/                                                 @imatwawana
 /docs/sources/whatsnew/                                                      @imatwawana
 /docs/sources/developers/plugins/                                            @Eve832 @josmperez @grafana/plugins-platform-frontend @grafana/plugins-platform-backend
-/docs/sources/developers/plugins/introduction-to-plugin-development/backend/ @Eve832 @grafana/plugins-platform-backend
 
 # Backend code
 /go.mod @grafana/backend-platform