* Add Sections and MO(layer)/TG(layer) Example
Major changes:
1. Added sub-section headings to the portion before the examples.
2. Added a new Example 6, that allows MO(layer) and TG(layer) functionality to be embedded within tap dance functions.
Minor Changes:
1. Edited some text to better fit with new sub-headings.
* Update feature_tap_dance.md
* Update feature_tap_dance.md
* add a tapdance example for creating advanced mod-tap and layer-tap keys
* add optional curly braces to match QMK conventions
* change example to use `register_code16()` and tapdance keycodes more closely matching QMK variants
* Fix some minor errors in tap dance example
Fix for #3529
Fix minor errors in the code examples for __Example 4: 'Quad Function Tap-Dance'__ and relevant documentation.
Clarified the need to include the header file in `keymap.c`.
* Use #pragma once in header guard
Fix for #3529
Implement change requested in #3530
I'm almost 100% sure "else if (state->count = 2) {" was a typo (it should have two ='s for a logical operator), and I'm *pretty* sure "if (state->interrupted || state->!pressed) return SINGLE_TAP;" has a typo. At least, it returns an error on my machine saying something about an unexpected '!'.
I changed it to a slightly longer form (i.e., "state->pressed==0"), and that worked fine.
