doc: updated links (close #709) and TODO file

TODO

@@ -10,6 +10,7 @@ WIP:
 
 WIP:
 * page size: split sparse/packed sizes, reduce comp page size, add per-pool size, allow for 0 sizes (old fully packed array)
+* make it possible to register externally managed pools with the registry (allow for system centric mode)
 * registry: switch to the udata/mixin model and get rid of poly storage, use pointer to sparse set only for pools, discard pool_data type.
 * make pools available (registry/view/group), review operator| for views
 * compressed pair to exploit ebo in sparse set and the others

docs/md/links.md

@@ -114,6 +114,8 @@ I hope this list can grow much more in the future:
     can run on a toaster.
   * [Helena Framework](https://github.com/NIKEA-SOFT/HelenaFramework): a modern
     framework in C++17 for backend development.
+  * [Unity/EnTT](https://github.com/TongTungGiang/unity-entt): tech demo of a
+    native simulation layer using `EnTT` and `Unity` as a rendering engine.
 
 * Articles, videos and blog posts:
   * [Some posts](https://skypjack.github.io/tags/#entt) on my personal