Add documentation about Undefined Behavior (#6302)

## Description

Add a section in the docs to list known UB.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

docs/book/src/SUMMARY.md

@@ -70,6 +70,7 @@
   - [Attributes](./reference/attributes.md)
   - [Style Guide](./reference/style_guide.md)
   - [Known Issues and Workarounds](./reference/known_issues_and_workarounds.md)
+  - [Behavior Considered Undefined](./reference/undefined_behavior.md)
   - [Differences From Solidity](./reference/solidity_differences.md)
   - [Differences From Rust](./reference/rust_differences.md)
   - [Contributing To Sway](./reference/contributing_to_sway.md)

docs/book/src/reference/undefined_behavior.md

@@ -0,0 +1,15 @@
+# Behavior Considered Undefined
+
+Sway code that contains any of the following behavior is considered undefined.
+The compiler is allowed to treat undefined Sway code however it desires,
+including removing it or replacing it with any other Sway code.
+
+This is not an exhaustive list, it may grow or shrink, there is no formal model
+of Sway's semantics so there may be more behavior considered undefined. We
+reserve the right to make some of the listed behavior defined in the future.
+
+* Invalid arithmetic operations (overflows, underflows, division by zero, etc)
+* Misuse of compiler intrinsics
+* Incorrect use of inline assembly
+* Reading and writing `raw_ptr` and `raw_slice`
+* Slicing and indexing out of bounds by directly using compiler intrinsics.