add intro to indexes per topic

doc/topics/index.md

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+# Indexes per Topic
+
+**Indexes per Topic** gather all resources already published by GitLab
+related to an specific subject or theme, including:
+  - General Docs
+  - Documentation Articles
+  - Blog Posts
+  - Video Tutorials
+
+## Topics
+
+- [Continuous Integration (GitLab CI)](ci/)
+- [GitLab Pages](pages/)
+- [Idea to Production](idea-to-production/)
+  - [GitLab Installation](idea-to-production/installation/)
+  - [GitLab Workflow](idea-to-production/workflow/)
+- [Authentication](authentication/)
+- [Deployment from GitLab](deployment/)
+- [Integrations](integration/)
+- [GitLab Flow (branching strategy)](gitlab-flow/)
+
+> Note: indexes currently under development. Will be available soon.