M1W: Enable bootmagic and update bootloader instructions

[Ace4896]

Hi, I recently bought a Monsgeek M1W V3 VIA. I found that after flashing the QMK firmware in this repository, I was unable to enter the bootloader anymore using Fn + Esc. This is because bootmagic is disabled by default when using info.json, and it wasn't explicitly enabled by the rules.mk file. In the end, I had to disassemble my M1W and short the BOOT + VCC pins to regain bootloader access.

I hope these changes help anyone else looking to use this QMK firmware.

Description

• Enable bootmagic, so it's possible to reset to bootloader by plugging in the keyboard while holding Esc
• Map Fn + Esc to QK_BOOT, so it's possible to reset to bootloader using this key combination
• Update documentation to reflect the available reset methods

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• N/A

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).