Update newbs_building_firmware.md : Guide users whose keymaps are in .json format.

[NBeing]

Add information regarding keyboards whose default keymaps are .json

Description

This documentation update will better guide users with keyboards containing default keymaps in the keymap.json format towards generating a keymap.c file.

Types of Changes

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

Issues Fixed or Closed by This PR

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).

[plodah]

suggest linking json2c to the relevant docs page;

[`json2c`](cli_commands#qmk-json2c)