Update the release cycle

doc/manual/src/SUMMARY.md.in

@@ -119,7 +119,7 @@
   - [Experimental Features](contributing/experimental-features.md)
   - [CLI guideline](contributing/cli-guideline.md)
   - [C++ style guide](contributing/cxx.md)
-- [Release Notes](release-notes/index.md)
+- [Releases](release-notes/index.md)
 {{#include ./SUMMARY-rl-next.md}}
   - [Release 2.21 (2024-03-11)](release-notes/rl-2.21.md)
   - [Release 2.20 (2024-01-29)](release-notes/rl-2.20.md)

doc/manual/src/release-notes/index.md

@@ -1,12 +1,15 @@
 # Nix Release Notes
 
-Nix has a release cycle of roughly 6 weeks.
-Notable changes and additions are announced in the release notes for each version.
+The Nix release cycle is calendar-based as follows:
+
+- A new minor version (`XX.YY+1.0`) is published every month and supported for two months;
+- A new major version (`XX+1.1.0`) is published twice a year, in April and October, and supported for eight months.
 
-Bugfixes can be backported on request to previous Nix releases.
-We typically backport only as far back as the Nix version used in the latest NixOS release, which is announced in the [NixOS release notes](https://nixos.org/manual/nixos/stable/release-notes.html#ch-release-notes).
+The rationale behind that cycle is that
+- Minor versions stay close to master and bring early access to new features for the user who need them;
+- Major versions are aligned with the NixOS releases (released one month before NixOS and supported for as long at it).
 
-Backports never skip releases.
-If a feature is backported to version `x.y`, it must also be available in version `x.(y+1)`.
-This ensures that upgrading from an older version with backports is still safe and no backported functionality will go missing.
+Bugfixes and security issues are backported to every supported version.
+Patch releases are published as needed.
 
+Notable changes and additions are announced in the release notes for each version.