add entities.raycast & update docs structure

doc/en/scripting/builtins/libentities.md

@@ -0,0 +1,26 @@
+# Library *entities*
+
+The library is designed to work with a registry of entities.
+
+```lua
+-- Returns an entity by unique identifier
+-- The table returned is the same one available in the entity components.
+entities.get(uid: int) -> table
+
+-- Creates the specified entity.
+-- args - table of component parameter tables (ARGS variable)
+-- args is optional
+entities.spawn(name: str, pos: vec3, [optional] args: table)
+
+-- Checks the existence of an entity by a unique identifier.
+entities.exists(uid: int) -> bool
+```
+
+```lua
+entities.raycast(start: vec3, dir: vec3, max_distance: number,
+ ignore: int, [optional] destination: table) -> table or nil
+```
+
+The function is an extended version of [block.raycast](libblock.md#raycast). Returns a table with the results if the ray touches a block or entity.
+
+Accordingly, this will affect the presence of the *entity* and *block* fields.