Add The Railroad keyboard and documentation follow-up (9942)

[nopunin10did]

Description

1. Add firmware code and documentation for The Railroad keyboard.
2. Fill in missed bits of documentation for LSA, RSA, RCS, etc. (follow-up to issue [Feature Request] Add mod key shortcuts and "_T" functions for multiple modifier combinations #9942 & PR Issue 9942: Add LSA, RSA, RCS, LSA_T, RSA_T, and RCS_T #9943)

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

Issue 9942 is already closed, but I realized that I'd missed some documentation files that needed similar updates.
Railroad-related changes are unrelated to 9942.

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).

[ridingqwerty]

I'd've submitted the documentation update and keyboard as separate PRs, but I think we can roll with it?

However, you're going to have to remove the submodule updates (lib/vusb/) from this PR.

[nopunin10did]

I'd've submitted the documentation update and keyboard as separate PRs, but I think we can roll with it?

However, you're going to have to remove the submodule updates (lib/vusb/) from this PR.

I'll remove them.

[nopunin10did]

I'm closing this PR. Changes have been split off to PR #10474 and PR #10473 .