[Keymap] Various improvements to my Lily58 keymap (#7045)

* Fix off-by-one error in Lily58 function keys

* Swap number and symbol layers

* Move grave/tilde to the left of brackets/braces

* Add KLE links

* Move function keys to Raise layer

* Move symbols nearer to home row

* Add readme for Lily58 layout

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+# bcat's Lily58 layout
+
+This split ergo layout is standard QWERTY on the default layer, with symbols on
+the lower layer, numbers/navigation on the raise layer, and media keys centered
+around the WASD cluster in the adjust (raise + lower) layer. A few general
+principles went into this layout:
+
+* The number row is optional, as are the outermost bottom row keys and the
+"extra" keys on the innnermost columns below the controllers. This will enable
+easy translation to a Crkbd or similar 40% ortho if I go that route.
+
+* Space and Enter are on the big thumb keys so they're easy to press
+(particularly with G20 or other low-profile keycaps there).
+
+* There are two shift keys because really, Right Shift is important.
+
+* Arrow keys are on VIM-style HJKL keys.
+
+* Home/End and Page Up/Page Down are easy to reach and in a square kind of like
+on the HHKB.
+
+* Escape is easy to reach because Vim.
+
+* Since most of the modifiers are on the left half, keys frequently pressed
+together with mods (e.g., numbers, function keys, etc.) are on the Raise layer
+activated by the right thumb.
+
+* Every key on a TKL has a binding.
+
+* Backspace is bound in the same place on every layer to avoid having to let go
+of layer-shift keys to fix a mistake.
+
+* Brackets and braces are on or near the home row for quick access when coding.
+
+* Other than Right Shift (sorry), modifiers aren't rebound on layers.
+
+## Default layer
+
+![Default layer layout](https://i.imgur.com/NjIp4Qr.png)
+
+([KLE](http://www.keyboard-layout-editor.com/#/gists/e0eb3af65961e9fd612dcff3ddd88e4f))
+
+## Lower layer
+
+![Lower layer layout](https://i.imgur.com/FyioHWW.png)
+
+([KLE](http://www.keyboard-layout-editor.com/#/gists/19ad0d3b5d745fbb2818db09740f5a11))
+
+## Raise layer
+
+![Raise layer layout](https://i.imgur.com/8Mrz4FF.png)
+
+([KLE](http://www.keyboard-layout-editor.com/#/gists/912be7955f781cdaf692cc4d4c0b5823))
+
+## Adjust layer
+
+![Adjust layer layout](https://i.imgur.com/FRKNqZp.png)
+
+([KLE](http://www.keyboard-layout-editor.com/#/gists/8f6a3f08350a9bbe1d414b22bca4e6c7))