Add versioning policy to documentation (#7923)

Most of the content adapted from
https://github.com/astral-sh/ruff/discussions/6998

docs/versioning.md

@@ -0,0 +1,61 @@
+# Versioning
+
+Ruff uses a custom versioning scheme that uses the **minor** version number for breaking changes and the **patch** version number for bug fixes. Ruff does not yet have a stable API; once Ruff's API is stable, the **major** version number and semantic versioning will be used.
+
+## Version changes
+
+**Minor** version increases will occur when:
+
+- A deprecated option or feature is removed
+- Configuration changes in a backwards incompatible way
+    - This _may_ occur in minor version changes until `1.0.0`, however it should generally be avoided.
+- A rule is promoted to stable
+- Support for a new file type is promoted to stable
+- Support for an end-of-life Python version is dropped
+- The behavior of a stable rule is changed
+    - Does not include bug fixes that follow the original intent of the rule
+- Stable rules are added to the default set
+- Stable rules are removed from the default set
+- A safe fix for a rule is promoted to stable
+
+**Patch** version increases will occur when:
+
+- Bugs are fixed, _including behavior changes that fix bugs_
+- An unsafe fix for a rule is added
+- A safe fix for a rule is added in preview
+- A fix’s applicability is demoted
+- A new configuration option is added
+- A rule is added in preview
+- The behavior of a preview rule is changed
+- Support for a new Python version is added
+- Support for a new file type is added in preview
+- An option or feature is deprecated
+
+## Preview mode
+
+A preview mode is available to enable new, unstable rules and features e.g. support for a new file type.
+
+The preview mode is intended to help us collect community feedback and gain confidence that changes are a net-benefit.
+
+The preview mode is _not_ intended to gate access to work that is incomplete or features that we are _likely to remove._ However, **we reserve the right to make changes to _any_ behavior gated by the mode** including the removal of preview features or rules.
+
+## Rule stabilization
+
+When modifying or adding rules, we use the following guidelines:
+
+- New rules are always be added in a preview mode
+- New rules remain in preview mode for at least one minor release before being promoted to stable
+    - If added in a patch release i.e. `0.6.1` then a rule are not be eligible for stability until `0.8.0`
+- Stable rule behavior are not changed significantly in patch versions
+- Promotion of rules to stable may be delayed in order to “batch” them into a single minor release
+- Not all rules in preview need to be promoted in a given minor release
+
+## Fix stabilization
+
+Fixes have three applicability levels:
+
+- **Display**: Never applied, just displayed.
+- **Unsafe**: Can be applied with explicit opt-in.
+- **Safe**: Can be applied automatically.
+
+Fixes for rules may be introduced at a lower applicability then promoted to a higher applicability. Reducing the applicability of a fix is not a breaking change. The applicability of a given fix may change when the preview mode is enabled.

scripts/generate_mkdocs.py

@@ -29,6 +29,7 @@ SECTIONS: list[Section] = [
     Section("Rules", "rules.md", generated=True),
     Section("Settings", "settings.md", generated=True),
     Section("Editor Integrations", "editor-integrations.md", generated=False),
+    Section("Versioning", "versioning.md", generated=False),
     Section("FAQ", "faq.md", generated=False),
     Section("Contributing", "contributing.md", generated=True),
 ]