docs: document backwards compatibility (#140)

Document backwards compatability based on the grafana/agent backwards
compatability [RFC]. Some changes have been made:

* Major release burnout and LTS suppor for previous major releases is
  not mentioned.

* "The scope of backwards compatability" has been removed from the
  backwards compatability list, as it is needlessly verbose and we may
  have forgotten an obvious exception.

* The exceptions "undocumented behavior," "tagged as exempt," "other
  telemetry data," and "non-versioned network APIs" have been integrated
  elsewhere in the doc to keep the list of exceptions shorter.

Despite the changes above, the intent is still the same: we strive
wherever possible to maintain compatability for users between minor and
patch releases.

[RFC]: https://github.com/grafana/agent/blob/main/docs/rfcs/0008-backwards-compatibility.md

Co-authored-by: Paulin Todev <[email protected]>
Co-authored-by: Clayton Cornell <[email protected]>
(cherry picked from commit 38fd764)

docs/sources/introduction/backward-compatability.md

@@ -0,0 +1,39 @@
+---
+canonical: https://grafana.com/docs/alloy/latest/introduction/backward-compatability/
+description: Grafana Alloy backward compatibility
+menuTitle: Backward compatibility
+title: Grafana Alloy backward compatibility
+weight: 999
+---
+
+# {{% param "FULL_PRODUCT_NAME" %}} backward compatibility
+
+{{< param "FULL_PRODUCT_NAME" >}} follows [semantic versioning][].
+{{< param "PRODUCT_NAME" >}} is stable, and we strive to maintain backward compatibility between minor and patch versions.
+
+Documented functionality that's released as _Generally available_ is covered by backward compatibility, including:
+
+* **User configuration**, including the {{< param "PRODUCT_NAME" >}} configuration syntax, the semantics of the configuration file, and the command-line interface.
+
+* **APIs**, for any network or code API released as v1.0.0 or later.
+
+* **Observability data used in official dashboards**, where the official set of dashboards are found in [the `alloy-mixin/` directory][alloy-mixin].
+
+## Exceptions
+
+We strive to maintain backward compatibility, but there are situations that may arise that require a breaking change without a new major version:
+
+* **Security**: A security issue may arise that requires breaking compatibility.
+
+* **Legal requirements**: If we learn that exposed behavior violates a licensing or legal requirement, a breaking change may be required.
+
+* **Specification errors**: If a specification for a feature is found to be incomplete or inconsistent, fixing the specification may require a breaking change.
+
+* **Bugs**: If a bug is found that goes against the documented specification of that functionality, fixing the bug may require breaking compatibility for users who are relying on the incorrect behavior.
+
+* **Upstream changes**: Much of the functionality of {{< param "PRODUCT_NAME" >}} is built on top of other software, such as OpenTelemetry Collector and Prometheus. If upstream software breaks compatibility, we may need to reflect this in {{< param "PRODUCT_NAME" >}}.
+
+We try, whenever possible, to resolve these issues without breaking compatibility.
+
+[semantic versioning]: https://semver.org/
+[alloy-mixin]: https://github.com/grafana/alloy/tree/main/operations/alloy-mixin