Fix broken docs link to stacked branches

Update references to the Stacked vs. Dependent documentation to use the new
branch-management path instead of the old virtual-branches path.

- Update Svelte CreateBranchModal component link to the new URL so the UI
  points users to the correct documentation.
- Update Rust crate lib docs to reference the new documentation location so
  generated docs and inline comments link correctly.

Author: jtaavola

apps/desktop/src/components/CreateBranchModal.svelte

@@ -232,7 +232,7 @@
 				>See more: <Link
 					target="_blank"
 					rel="noreferrer"
-					href="https://docs.gitbutler.com/features/virtual-branches/stacked-branches"
+					href="https://docs.gitbutler.com/features/branch-management/stacked-branches"
 					>Stacked vs. Dependent</Link
 				></span
 			>

crates/but-workspace/src/lib.rs

@@ -14,7 +14,7 @@
 //!   - GitButler implements the concept of a branch stack. This is essentially a collection of "heads"
 //!     (pseudo branches) that contain each other.
 //!   - Always contains at least one branch.
-//!   - High level documentation here: <https://docs.gitbutler.com/features/virtual-branches/stacked-branches>
+//!   - High level documentation here: <https://docs.gitbutler.com/features/branch-management/stacked-branches>
 //! * **Target Branch**
 //!   - The branch every stack in the workspace wants to get merged into.
 //!   - It's usually a local tracking branch, but doesn't have to if no Git *remote* is associated with it.