[Keymap] add ai03/polaris:mekberg (#10508)

* Add keymap for Polaris

* Change back to KC_NUBS for <> key

* Add GPL header from default files

* Change to new way of flashing in readme

keyboards/ai03/polaris/keymaps/mekberg/readme.md

@@ -0,0 +1,14 @@
+# General Information
+
+This is more or less a HHKB base layout, but with completely different Fn layers. I don't use most of the HHKB secondary key positions because, honestly, they make no sense to me at all.
+
+Fn key layer is focused mostly on keyboard firmware features (like RGB) as well as some of the necessary alternate functions. It also hold media controls, F-keys and Reset.
+
+Left Tab-Hold layer is focused on nav cluster functionality, and turns Backspace into Delete (and moves Backspace up into the top right 1u position).
+
+
+# Build instructions
+
+To simply build the firmware file: `make clean && make ai03/polaris:mekberg`
+
+To build and immediately flash: `make clean && make ai03/polaris:mekberg:flash`