Add a logistic regression example to the Getting Started section

[Saransh-cpp]

This PR adds a new Logistic Regression example to the "Getting Started" section.

Note: This PR contains the changes from the linear regression PR (without some of these changes, the tests would fail). This PR should be merged after the linear regression example, and I will make sure to sync the changes before this PR is merged.

Rendered guide - https://github.com/Saransh-cpp/Flux.jl/blob/logistic-regression/docs/src/getting_started/logistic_regression.md

PR Checklist

• [ ] Tests are added
• [ ] Entry in NEWS.md
• [ ] Documentation, if applicable

[Saransh-cpp]

~TODO: switch to OneHotArrays.jl~

[Saransh-cpp]

Rebased and fixed the doctests!