Add Gyro feature documentation

[gonzalo-bulnes]

Overview

Adds documentation for the upcoming Gyro feature.

See work in progress for reference:

• https://github.com/frankiearzu/ExpressLRS/tree/gyro-4.1-merge (RX code, Lua script, docs)
• https://github.com/pkendall64/ExpressLRS/commits/wifi-config-mode (WebUI)

Note: I'll keep history linear for now, and squash commits when needed.

Note 2: TK is a placeholder for missing text or images. I'm intentionally not adding it to the spell checker wordlist.

To do

The structure of the docs is in place, and most of the sections are ready for review. Below is the list of remaining work.

Software > Gyro

• Advanced (PIDs) section (covers gyro mode advanced PID tuning)
• the WebUI detail instructions have screenshots
• the Lua script detail instructions have screenshots
  I went for a text representation of the Lua script (easier to maintain), that matches what a B&W displays shows. We can add a tab for the color version if that seems useful, but the text content of both does match, so IMHO this covers well both.

Hardware > Receiver Gyro Mod

• Receiver configuration section (covers Hardware and Connections tabs of the WebUI)
• Wiring section, SPI insert
  [ ]Testing` section (covers what to look for before moving to the Software > Gyro docs
• the WebUI detail instructions have screenshots
• the Lua script detail instructions have screenshots

[gonzalo-bulnes]

It occurred to me that having a live preview of the current (total) gain (when the TX is connected) could be quite useful around here. As a way to confirm that the gyro gain channel is properly set up, but also a way to experiment with the rate gain multiplier and the stick sensitivity settings and "see" rather than read how the work.

By gain I mean the multiplier that's common to all functions, before their specific gains. (Rate gain multiplier x multiplier(stick sensitivity, stick position) ... that varies by function, darn, I was thinking aloud but it doesn't quite work. Leaving it here in case you think of something.)