doc: cleanup documentation relating to release notes

Also delete empty release notes file.

.github/CONTRIBUTING.md

@@ -70,6 +70,8 @@ Anything that does not cause user or downstream dependency regressions can be ba
 
 ## Generating 21.11 Release Notes
 
+(This section also applies to backporting 21.05 release notes: substitute "rl-2111" for "rl-2105".)
+
 Documentation in nixpkgs is transitioning to a markdown-centric workflow. Release notes now require a translation step to convert from markdown to a compatible docbook document.
 
 Steps for updating 21.11 Release notes:

.github/PULL_REQUEST_TEMPLATE.md

@@ -23,7 +23,7 @@ Reviewing guidelines: https://nixos.org/manual/nixpkgs/unstable/#chap-reviewing-
 - [ ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
 - [ ] Tested compilation of all pkgs that depend on this change using `nix-shell -p nixpkgs-review --run "nixpkgs-review wip"`
 - [ ] Tested execution of all binary files (usually in `./result/bin/`)
-- [21.11 Release Notes](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md#generating-2111-release-notes)
+- [21.11 Release Notes (or backporting 21.05 Relase notes)](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md#generating-2111-release-notes)
   - [ ] (Package updates) Added a release notes entry if the change is major or breaking
   - [ ] (Module updates) Added a release notes entry if the change is significant
   - [ ] (Module addition) Added a release notes entry if adding a new NixOS module