Free en course elastic fix (#10845)

* Update free-programming-books-langs.md

* Update free-programming-books-langs.md

Adjust Spacing between headers and sections

* Update free-programming-playgrounds.md with Gremlin

Add Gremlin Playground reference

* Update free-programming-playgrounds.md

* Add gremlin cheatsheets and samples

* Fix Elastic Security fundamentals reference

* Revert Changes from previous PR.

courses/free-courses-en.md

@@ -665,7 +665,7 @@
 * [App Search Fundamentals](https://www.elastic.co/training/app-search-fundamentals) - Elastic
 * [App Search Web Crawler Fundamentals](https://www.elastic.co/training/app-search-web-crawler-fundamentals) - Elastic
 * [ECE Fundamentals](https://www.elastic.co/training/ece-fundamentals) - Elastic
-* [Elastic Security Fundamentals: SIEM]([https://www.elastic.co/training/kibana-for-splunk-spl-users](https://www.elastic.co/training/elastic-security-fundamentals-siem)) - Elastic
+* [Elastic Security Fundamentals: SIEM](https://www.elastic.co/training/elastic-security-fundamentals-siem) - Elastic
 * [Kibana for Splunk SPL Users](https://www.elastic.co/training/kibana-for-splunk-spl-users) - Elastic
 * [Kibana Fundamentals](https://www.elastic.co/training/kibana-fundamentals) - Elastic
 * [Observability Fundamentals](https://www.elastic.co/training/observability-fundamentals) - Elastic