Update inline forms (updated docs and new flexbox styles)

[mdo]

Supersedes #19129, fixes #18406.

This branch takes the work started by @wojtask9 and builds on it heavily to address some inline form styling issues, for the default and flexbox modes. Specifically:

• Provides flexbox support for inline forms, including inputs, input groups, selects, checks, custom selects, custom checks, and buttons.
• Adjusts the default mode styling for alignment of (custom) checks.
• Updates the docs to remove the hidden/visible label demos and replace them with more complete examples featuring all control types and margin utils for spacing.
• Updates the usage guidelines to mention flexbox and crosslink some stuff.

Here's how the docs look now.

[houndci-bot]

Properties should be ordered display, flex-direction, margin-bottom, justify-content