docs: add instructions about how to write a `.dp-in` file.

[ycexiao]

Problem

Add instructions about how to .dp-in file like:

load structure G1 from "{str(Path(__file__).parents[3] / "tests/data/Ni.cif")}"
load profile exp_ni from "{str(Path(__file__).parents[3] / "tests/data/Ni.gr")}"

set G1 spacegroup as auto
set exp_ni q_range as 0.1 25
set exp_ni calculation_range as 1.5 50 0.01
create equation variables s0
set equation as "s0*G1"
save to "results.json"

variables:
---
- G1.a: 3.52
- s0: 0.4
- G1.Uiso_0: 0.005
- G1.delta2: 2
- qdamp: 0.04
- qbroad: 0.02
---

Proposed solution