Doc: add more description for Template keyword

[TonyHo]

Any place have the info about the meaning of all kinds of Template keywords?

For example:

const simplestTemplate = "[{{datetime}}] [{{level}}] {{message}} {{data}} {{extra}}"

What's the meaning of data and extra in above template string?

Thanks.

[Inhere]

data, extra - Used to logging some complex data

examples:

slog.Warn("warning log message")

slog.WithData(slog.M{
		"key0": 134,
		"key1": "abc",
}).Infof("info log %s", "message")

output:

[WARNING] warning log message
[INFO] info log message {key0:134, key1:abc}