feat: add information about show tables in docs

Author: jossephus

docs/statement/variables.md

@@ -12,4 +12,4 @@ You can use the variable like any other symbol using the name
 
 ```sql
 SELECT @one
-```
+```

docs/structure/tables.md

@@ -1,4 +1,12 @@
-## GQL Tables 
+## GQL Tables
+
+You can see list of all tables directly in the repl using
+
+```sql
+show tables
+
+```
+
 ---
 
 ### References table
@@ -57,4 +65,4 @@
 | Name | Type | Description          |
 | ---- | ---- | -------------------- |
 | name | Text | Tag name             |
-| repo | Text | Repository full path |
+| repo | Text | Repository full path |