[Mini-Roadmap] Irrationality

## Goal
Сomplete the list of open questions and conjectures on irrational and transcendental numbers.

## Plan
- [ ] Update `FormalConjectures/Wikipedia/Irrational.lean`
- [ ] Make a list for open questions on transcendence
- [ ] Algebraic independence
- [x] Add [Schanuel's conjecture](https://en.wikipedia.org/wiki/Schanuel%27s_conjecture)
- [x] ~~Add [Littlewood conjecture](https://en.wikipedia.org/wiki/Littlewood_conjecture)~~
- [ ] ~~Add conjecture about $\mathrm{SL}_n$ from [Littlewood conjecture](https://en.wikipedia.org/wiki/Littlewood_conjecture)~~
- [ ] Add family of [exponentials conjectures](https://en.wikipedia.org/wiki/Four_exponentials_conjecture)
- [ ] [Erdős 68](https://www.erdosproblems.com/68)

I will be glad to add more related conjectures to the list!

## Suggestions
- Allocate a separate folder for this topic `FormalConjectures/Irrational`
- Merge `Irrational.lean` and `RiemannZeta.lean`