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/src/fixed_point.rs

@@ -16,6 +16,33 @@
 // limitations under the License.
 
 //! Decimal Fixed Point implementations for Substrate runtime.
+//! Similar to types that implement [`PerThing`](crate::per_things), these are also
+//! fixed-point types, however, they are able to represent larger fractions:
+#![doc = docify::embed!("./src/lib.rs", fixed_u64)]
+//!
+//! ### Fixed Point Types in Practice
+//!
+//! If one needs to exceed the value of one (1), then
+//! [`FixedU64`](FixedU64) (and its signed and `u128` counterparts) can be utilized.
+//! Take for example this very rudimentary pricing mechanism, where we wish to calculate the demand
+//! / supply to get a price for some on-chain compute:
+#![doc = docify::embed!(
+	"./src/lib.rs",
+	fixed_u64_block_computation_example
+)]
+//!
+//! For a much more comprehensive example, be sure to look at the source for broker (the "coretime")
+//! pallet.
+//!
+//! #### Fixed Point Types in Practice
+//!
+//! Just as with [`PerThing`](PerThing), you can also perform regular mathematical
+//! expressions:
+#![doc = docify::embed!(
+	"./src/lib.rs",
+	fixed_u64_operation_example
+)]
+//!
 
 use crate::{
 	helpers_128bit::{multiply_by_rational_with_rounding, sqrt},