Improve documentation for `excludeBySource`

[klezm]

As discussed in #151, #126 and #79 the function of excludeBySource doesn't seem to be very clear.

I'd suggest to update the settings description and add examples for different languages to the README.

settings.json:

{
    ...
    "errorLens.excludeBySource": [
        // Python
        "Ruff", // Disables all errors reported by the `Ruff` linter
        "Ruff(E501)", // Disables errors by the `Ruff` linter with the code `E501` (Line to long)
        "Pylance(reportUndefinedVariable)",

        // JavaScript
        "eslint(padded-blocks)",
    ]
}