Improve unit conversion API to be more accessible outside of formatting

[hl662]

For users outside of the repo, calculating unit conversion between 2 units can be simplified.

Here is a code example of how users can do it now:

import { Quantity } from "@itwin/core-quantity";

const context = new SchemaContext(); // or from iModelDb.schemaContext

const provider = new SchemaUnitProvider(context);

const fromUnit = await provider.findUnitByName("Units.M");
const toUnit = await provider.findUnitByName("Units.FT");
const conversion = await provider.getConversion(fromUnit, toUnit);

const quantity = new Quantity(fromUnit, 1.0);
const converted = quantity.convertTo(toUnit, conversion);
console.log(converted?.magnitude);

It involves instantiating a Quantity object and calling the convertTo method, a wrapper around an internal applyConversion method in core-quantity. We should look into

1. An easy way to convert values between units without needing an instanced object (which can mean making applyConversion not internal)
2. Improving the documentation in quantity to explicitly have a code example for external developers to convert values between 2 units outside of formatting.

[hl662]

After reviewing the unit conversion API, there's not a lot more we could do in terms of promoting internal APIs to make unit conversion more accessible than it is. applyConversion has no validation/enforcement between 2 units to check if they match the same phenomenon, so anyone who doesn't understand units and phenomena can get incorrect unit conversions, so I'd like to keep it internal.

Instead, through #8963 and #8859, we've improved the documentation with proper code examples and a dedicated page for unit conversions, as well as fix inconsistent typings, this helps developers adopt unit conversion apis separately from formatting, so I'm closing this issue.