refactor: re-organize docs

mkdocs.yml

+---
+
 site_name: ANIS Documentation
 dev_addr: 0.0.0.0:2080
 theme: readthedocs
 extra_css: [extra.css]
 nav:
-    - Overview: index.md
-    - Installation: installation.md
-    - Metamodel database: metamodel_database.md
-    - Authorization: authorization.md
-    - Data management: data_management.md
-    - Data search: data_search.md
-    - Server API: server_api.md
-    - Web portal: web_portal.md
-    - Web admin: web_admin.md
-    - Web client: web_client.md
\ No newline at end of file
+    - Overview:
+          - What is ANIS?: index.md
+          - Installation: installation.md
+    - ANIS Configuration Basics:
+          - Metamodel database: metamodel_database.md
+          - Server API: server_api.md
+          - File management: data_management.md
+          - Authorization management: authorization.md
+    - The web interface of ANIS:
+          - Web portal: web_portal.md
+          - Web admin: web_admin.md
+          - Web client: web_client.md
+    - ANIS Scientific API:
+          - REST API description: data_search.md