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>

substrate/primitives/arithmetic/Cargo.toml

@@ -26,6 +26,8 @@ num-traits = { version = "0.2.17", default-features = false }
 scale-info = { version = "2.10.0", default-features = false, features = ["derive"] }
 serde = { features = ["alloc", "derive"], optional = true, workspace = true }
 static_assertions = "1.1.0"
+sp-std = { path = "../std", default-features = false }
+docify = "0.2.7"
 
 [dev-dependencies]
 criterion = "0.4.0"
@@ -41,6 +43,7 @@ std = [
 	"scale-info/std",
 	"serde/std",
 	"sp-crypto-hashing/std",
+  "sp-std/std",
 ]
 # Serde support without relying on std features.
 serde = ["dep:serde", "scale-info/serde"]