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>

Cargo.lock

@@ -13376,7 +13376,9 @@ dependencies = [
  "pallet-assets",
  "pallet-aura",
  "pallet-authorship",
+ "pallet-babe",
  "pallet-balances",
+ "pallet-broker",
  "pallet-collective",
  "pallet-default-config-example",
  "pallet-democracy",
@@ -13385,6 +13387,7 @@ dependencies = [
  "pallet-examples",
  "pallet-multisig",
  "pallet-proxy",
+ "pallet-referenda",
  "pallet-scheduler",
  "pallet-timestamp",
  "pallet-transaction-payment",
@@ -13404,6 +13407,7 @@ dependencies = [
  "scale-info",
  "simple-mermaid",
  "sp-api",
+ "sp-arithmetic",
  "sp-core",
  "sp-io",
  "sp-keyring",
@@ -18395,6 +18399,7 @@ name = "sp-arithmetic"
 version = "23.0.0"
 dependencies = [
  "criterion 0.4.0",
+ "docify 0.2.7",
  "integer-sqrt",
  "num-traits",
  "parity-scale-codec",
@@ -18403,6 +18408,7 @@ dependencies = [
  "scale-info",
  "serde",
  "sp-crypto-hashing",
+ "sp-std 14.0.0",
  "static_assertions",
 ]