Defensive Programming in Substrate Reference Document (#2615)

_This PR is being continued from
https://github.com/paritytech/polkadot-sdk/pull/2206, which was closed
when the developer_hub was merged._
closes https://github.com/paritytech/polkadot-sdk-docs/issues/44

---
# Description

This PR adds a reference document to the `developer-hub` crate (see
https://github.com/paritytech/polkadot-sdk/pull/2102). This specific
reference document covers defensive programming practices common within
the context of developing a runtime with Substrate.

In particular, this covers the following areas: 

- Default behavior of how Rust deals with numbers in general
- How to deal with floating point numbers in runtime / fixed point
arithmetic
- How to deal with Integer overflows
- General "safe math" / defensive programming practices for common
pallet development scenarios
- Defensive traits that exist within Substrate, i.e.,
`defensive_saturating_add `, `defensive_unwrap_or`
- More general defensive programming examples (keep it concise)
- Link to relevant examples where these practices are actually in
production / being used
- Unwrapping (or rather lack thereof) 101

todo
-- 
- [x] Apply feedback from previous PR
- [x] This may warrant a PR to append some of these docs to
`sp_arithmetic`

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Radha <86818441+DrW3RK@users.noreply.github.com>

docs/sdk/src/reference_docs/mod.rs

@@ -47,8 +47,7 @@ pub mod signed_extensions;
 pub mod frame_origin;
 
 /// Learn about how to write safe and defensive code in your FRAME runtime.
-// TODO: @CrackTheCode016 https://github.com/paritytech/polkadot-sdk-docs/issues/44
-pub mod safe_defensive_programming;
+pub mod defensive_programming;
 
 /// Learn about composite enums and other runtime level types, such as "RuntimeEvent" and
 /// "RuntimeCall".