Provide an simple overview of what a layer is at the start of the layers table

At the start of https://docs.wildfly.org/21/Bootable_Guide.html#wildfly_layers a couple sentences stating what a layer is would be good. Assume that someone has followed a link to that section and is coming to it without much context. (I'm thinking about adding such a link to the WF 21 release announcement blog post, which is what inspired me to file this.)

That's shared content with other guides, so my intent is those couple sentences would be in the other guides too.

This is a bit tricky as it should be brief, clear and accurate. Doesn't need to be complete though; i.e. the idea is to give basic context, not a detail explanation.