Add documentation page for Income Tax

[chelsea128]

Thanks for contributing to OpenFisca! Please remove this line, as well as, for each line below, the cases which are not relevant to your contribution :)

• Tax and benefit system evolution. | Technical improvement. | Crash fix. | Minor change.
• Impacted periods: all. | until YYYY-MM-DD. | from YYYY-MM-DD.
• Impacted areas: path/to/file/containing/impacted/variables
• Details:
  • New feature or new behaviour description
  • Cases for which an error was noticed

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These changes (remove lines which are not relevant to your contribution):

• Impact the PolicyEngine-UK public API (for instance renaming or removing a variable)
• Add a new feature (for instance adding a variable)
• Fix or improve an already existing calculation.
• Change non-functional parts of this repository (for instance editing the README)

[nikhilwoodruff]

from policyengine_uk import Simulation

simulation = Simulation(
    situation=dict(
            people=dict(
            person=dict(
                age=30,
                dividend_income=10_000,
            )
        ),
        axes=[[
            dict(
                name="employment_income",
                min=0,
                max=200_000,
                count=1_000,
            )
        ]]
    ),
)

import plotly.express as px
import pandas as pd
from policyengine_core.charts import format_fig

df = pd.DataFrame({
    "Employment income": simulation.calculate("employment_income"),
    "Income tax": simulation.calculate("income_tax"),
    "Dividend income tax": simulation.calculate("dividend_income_tax"),
})

fig = px.line(
    df,
    x="Employment income",
    y=["Income tax", "Dividend income tax"],
    title="Income tax vs employment income",
    labels={
        "value": "Income tax (£)",
        "variable": "Tax type",
    },
)

format_fig(fig)