docs: add Release Candidates section to releases page (#23584)

docs/install/releases/index.md

@@ -9,12 +9,13 @@ deployment.
 
 ## Release channels
 
-We support four release channels:
+We support four primary release channels, as well as ad-hoc release candidates:
 
 - **Mainline:** The bleeding edge version of Coder
 - **Stable:** N-1 of the mainline release
 - **Security Support:** N-2 of the mainline release
 - **Extended Support Release:** Biannually released version of Coder
+- **Release Candidates:** Ad-hoc builds to validate in-development features
 
 We field our mainline releases publicly for one month before promoting them to stable. The security support version, so n-2 from mainline, receives patches
 only for security issues or CVEs.
@@ -49,6 +50,15 @@ ESR releases will be updated with critical bugfixes and security patches that ar
 
 For more information, see the [Coder ESR announcement](https://coder.com/blog/esr) or our [ESR Upgrade Guide](./esr-2.24-2.29-upgrade.md).
 
+### Release Candidates
+
+- Ad-hoc builds that Coder releases to validate in-development features with select customers
+- Not guaranteed to be stable or free of bugs
+- Features introduced in an RC are not guaranteed to be included in a mainline or stable release
+- Not intended for production use
+
+Release candidates give Coder a way to push out builds for customers and other users to try out new, under-development functionality without cutting a new minor version. Unlike mainline and stable releases, RCs do not follow a fixed schedule and carry no guarantees around stability or long-term support. They exist purely as a feedback mechanism: Coder can ship targeted builds, gather real-world input, and iterate before committing changes to the standard release channels.
+
 ## Installing stable
 
 When installing Coder, we generally advise specifying the desired version from