Remove coupling with Miryoku ZMK repository

[kucera-lukas]

This repo has originally started as a fork of the Miryoku ZMK repository (https://github.com/manna-harbour/miryoku_zmk). Since then I've made lots of changes to every Miryoku layer and overrode most of the logic via the custom_config.h file.

Since my changes have diverged so much from base Miryoku, it's only an extra overhead to keep the repo aligned with upstream structure. On the other hand, there's nothing wrong with the current setup and it works well -- it might take me a while to find the time to do this work.

List of items to be done:

• Remove "layer lock" functionality (u_to_U_*) -- I don't use it and it causes confusion
• Standardize build processes. Current build process is too generic for my use-case and isn't well integrated with the native way to use the various ZMK tooling: zmkfirmware/zmk/.github/workflows/build-user-config, https://github.com/urob/zmk-actions, https://github.com/urob/zmk-helpers, build.yaml, west.yaml, ... -- ultimately it might be the easiest to start from a fresh ZMK config repo (https://zmk.dev/docs/user-setup)
• Remove Miryoku "feature flags" (MIRYOKU_KLUDGE_DOUBLETAPBOOT, MIRYOKU_KLUDGE_MOUSEKEYSPR, ...)
• Remove alternative layers (and layer selection)
• Remove unused keyboards (definitions, github actions, keymaps, build examples, outboards, shields)
• Documentation (e.g. local/remote builds). Relates to Update README with personal notes #20