Missing API docs for APCA formula

## Description

Themes accept a `formula` argument, which also has support in setter to change from 'wcag2' to 'wcag3'. The 'wcag3' option uses the APCA formula for calculating contrast.

The feature already exists but does not appear to be documented in the README for the contrast-colors package.

## Steps to reproduce

## Expected behavior

## Screenshots

## Leonardo package and version

## Environment
- **Browser(s) and OS(s):**

## Additional context

Start in the contrast-colors package README and locate the theme API section. Document the existing formula option, including the wcag3 value and its APCA contrast calculation, along with the setter behavior for changing from wcag2. Done means the README clearly explains how to use this existing feature.

Written by the indexing model from the issue text.