[do not merge] Add nullbitsco tidbit board with pin mapping; doc updates; hex composer

[patricksurry]

The hex_compose module is experimental keyboard-side composer for ascii/unicode characters which i'll use to drive a retro pc. I can separate into a different PR if that's helpful

[jaygreco]

Looks good! There is only one change (keycode syntax) I needed to make to run this on my development TIDBIT with Bit-C PRO.

[jaygreco]

These will all need to be converted from QMK syntax (underscore) to KMK (dot):

keyboard.keymap = [
    [
        XXXXX,  KC.PSLS, KC.PAST, KC.PMNS,
        KC.P7,  KC.P8,   KC.P9,   KC.PPLS,
        KC.P4,  KC.P5,   KC.P6,   KC.PPLS,
        KC.P1,  KC.P2,   KC.P3,   KC.PENT,
        KC.P0,  KC.P0,   KC.PDOT, KC.PENT,
    ]
]

[patricksurry]

Splitting this into smaller PRs.

• Mapping for nullbits tidbit board with bit-c pro #916 has the vanilla tidbit stuff with Jay's suggestions.
• minor doc updates #917 has a few suggested doc changes
• [do not merge] Tidbit hexpad with keyboard-side hex character composition #918 has a more complex tidbit hexpad build with hex character composition (wip)

[xs5871]

There are significant overlaps between #916 and #918. What do you want me to do here? Would you like to split those into atomic, potentially dependent PRs? Also, please close PRs you don't want to be considered for merging.

[patricksurry]

Sorry, yes I will close this PR. Please go ahead and merge 916 (if it looks ok) and I'll rebase 918 and resubmit