Add clarifying comments and add new (not bound by default) GAME layer

[tintededges]

This PR adds a bit of clarifying text in the same style as the existing documentation, and a togglable gaming layer, which has no modkey functionality and contains WASD and other keys commonly needed for playing games.

I believe a separate layer is the best solution for gaming with a Miryoku keyboard, as e.g. using different layouts means you might not have WASD on your left hand, and importantly, regular keys have hold actions which interrupt gaming, like the space button, which doesn't register in games due to its modkey functionality.

I've seen a post of someone online trying to do the same, but they ended up changing and moving their entire modkey row to a less ergonomic position, this solution has no such compromises, and would be discoverable and understandable by anyone wishing to do the same.

The use-case for this kind of a layer means it doesn't make sense to make it a temporarily-switched-to layer like the others, it needs to be toggled, which goes against what seems to be the design philosophy and flow of Miryoku. That's the reason the layer doesn't have a binding by default.

If a togglable layer is considered an okay addition, I could suggest adding it to unused keys of the more obscure Button layer, like I've done on the personal branch of my fork, which is harder to accidentally press. Let me know and I can add this change as well.

If not, I still think an unused extra layer is a worthy addition, as the effort it takes to implement a new layer is relatively tedious, and requires trial-and-error. With this layer in the file, anyone wanting to add this functionality themselves can simply bind this extra layer, instead of possibly giving up on figuring out how to add a new layer. They could also easily repurpose this layer if they want to add a new layer for other reasons.

Of note, I couldn't find the toggle-layer commands for other layouts than ZMK and QMK, but a part of the added clarifying text also illuminates the need for layer changing commands to be supported in the keycode-translations table.

[manna-harbour]

Thanks for the expanded docs!

For customisation via custom_config.h and Miryoku Babel see manna-harbour/miryoku#85. Most users seem to customise via custom_config.h rather than Miryoku Babel. Also see https://github.com/manna-harbour/miryoku/tree/master/docs/reference#additional-features for the Extra and Tap layers that can be used as or substituted with game layers.

There might be scope to include a selection of game layers in Miryoku Babel, but they would probably only be of use for those few customising via Miryoku Babel and who don't want to customise their game layers.

Also, you're welcome to share ideas for game layers in Show and tell Discussions at https://github.com/manna-harbour/miryoku/discussions/categories/show-and-tell.