Fix StableMIR migration documentation (rust-lang#2915)

Rename the file to use '-' instead of `_` and add the new page to the
index. Fixed version can be seen here:
https://celinval.github.io/kani-dev/stable-mir.html

Author: celinval

Diff for: docs/src/SUMMARY.md

@@ -26,6 +26,7 @@
   - [Coding conventions](./conventions.md)
   - [Working with CBMC](./cbmc-hacks.md)
   - [Working with `rustc`](./rustc-hacks.md)
+  - [Migrating to StableMIR](./stable-mir.md)
   - [Command cheat sheets](./cheat-sheets.md)
   - [cargo kani assess](./dev-assess.md)
   - [Testing](./testing.md)

Diff for: docs/src/dev-documentation.md

@@ -14,7 +14,7 @@ developers (including external contributors):
  3. [Development setup recommendations for working with `cbmc`](./cbmc-hacks.md).
  4. [Development setup recommendations for working with `rustc`](./rustc-hacks.md).
  5. [Guide for testing in Kani](./testing.md).
- 6. [Transition to StableMIR](./stable_mir.md).
+ 6. [Transition to StableMIR](./stable-mir.md).
 
 > **NOTE**: The developer documentation is intended for Kani developers and not
 users. At present, the project is under heavy development and some items